JIT: Add successor edge iterators; remove old fgReplacePred (#99097)

Part of #93020. We already have successor block iterators, but being able to iterate by successor edge grants us easy access to the edges themselves (i.e. no fgGetPredForBlock required). This also allows us to get rid of the old version of fgReplacePred, which had to look up the successor edge.

To reduce code duplication between the successor block/edge iterators, I moved the members of BBSuccList into an abstract class so the corresponding edge iterator type can inherit from it.

src/coreclr/jit/block.cpp

@@ -1203,15 +1203,15 @@ unsigned BasicBlock::NumSucc() const
 }
 
 //------------------------------------------------------------------------
-// GetSucc: Returns the requested block successor. See the declaration comment for details.
+// GetSucc: Returns the requested successor edge. See the declaration comment for details.
 //
 // Arguments:
 //    i - index of successor to return. 0 <= i <= NumSucc().
 //
 // Return Value:
-//    Requested successor block
+//    Requested successor edge
 //
-BasicBlock* BasicBlock::GetSucc(unsigned i) const
+FlowEdge* BasicBlock::GetSuccEdge(unsigned i) const
 {
     assert(i < NumSucc()); // Index bounds check.
     switch (bbKind)
@@ -1222,31 +1222,45 @@ BasicBlock* BasicBlock::GetSucc(unsigned i) const
         case BBJ_EHCATCHRET:
         case BBJ_EHFILTERRET:
         case BBJ_LEAVE:
-            return GetTarget();
+            return GetTargetEdge();
 
         case BBJ_COND:
             if (i == 0)
             {
-                return GetFalseTarget();
+                return GetFalseEdge();
             }
             else
             {
                 assert(i == 1);
                 assert(bbTrueEdge != bbFalseEdge);
-                return GetTrueTarget();
+                return GetTrueEdge();
             }
 
         case BBJ_EHFINALLYRET:
-            return bbEhfTargets->bbeSuccs[i]->getDestinationBlock();
+            return bbEhfTargets->bbeSuccs[i];
 
         case BBJ_SWITCH:
-            return bbSwtTargets->bbsDstTab[i]->getDestinationBlock();
+            return bbSwtTargets->bbsDstTab[i];
 
         default:
             unreached();
     }
 }
 
+//------------------------------------------------------------------------
+// GetSucc: Returns the requested block successor. See the declaration comment for details.
+//
+// Arguments:
+//    i - index of successor to return. 0 <= i <= NumSucc().
+//
+// Return Value:
+//    Requested successor block
+//
+BasicBlock* BasicBlock::GetSucc(unsigned i) const
+{
+    return GetSuccEdge(i)->getDestinationBlock();
+}
+
 //------------------------------------------------------------------------
 // NumSucc: Returns the count of block successors. See the declaration comment for details.
 //
@@ -1314,16 +1328,16 @@ unsigned BasicBlock::NumSucc(Compiler* comp)
 }
 
 //------------------------------------------------------------------------
-// GetSucc: Returns the requested block successor. See the declaration comment for details.
+// GetSucc: Returns the requested successor edge. See the declaration comment for details.
 //
 // Arguments:
 //    i - index of successor to return. 0 <= i <= NumSucc(comp).
 //    comp - Compiler instance
 //
 // Return Value:
-//    Requested successor block
+//    Requested successor edge
 //
-BasicBlock* BasicBlock::GetSucc(unsigned i, Compiler* comp)
+FlowEdge* BasicBlock::GetSuccEdge(unsigned i, Compiler* comp)
 {
     assert(comp != nullptr);
 
@@ -1333,30 +1347,30 @@ BasicBlock* BasicBlock::GetSucc(unsigned i, Compiler* comp)
         case BBJ_EHFILTERRET:
             // Handler is the (sole) normal successor of the filter.
             assert(comp->fgFirstBlockOfHandler(this) == GetTarget());
-            return GetTarget();
+            return GetTargetEdge();
 
         case BBJ_EHFINALLYRET:
             assert(bbEhfTargets != nullptr);
             assert(i < bbEhfTargets->bbeCount);
-            return bbEhfTargets->bbeSuccs[i]->getDestinationBlock();
+            return bbEhfTargets->bbeSuccs[i];
 
         case BBJ_CALLFINALLY:
         case BBJ_CALLFINALLYRET:
         case BBJ_ALWAYS:
         case BBJ_EHCATCHRET:
         case BBJ_LEAVE:
-            return GetTarget();
+            return GetTargetEdge();
 
         case BBJ_COND:
             if (i == 0)
             {
-                return GetFalseTarget();
+                return GetFalseEdge();
             }
             else
             {
                 assert(i == 1);
                 assert(bbTrueEdge != bbFalseEdge);
-                return GetTrueTarget();
+                return GetTrueEdge();
             }
 
         case BBJ_SWITCH:
@@ -1371,6 +1385,21 @@ BasicBlock* BasicBlock::GetSucc(unsigned i, Compiler* comp)
     }
 }
 
+//------------------------------------------------------------------------
+// GetSucc: Returns the requested block successor. See the declaration comment for details.
+//
+// Arguments:
+//    i - index of successor to return. 0 <= i <= NumSucc(comp).
+//    comp - Compiler instance
+//
+// Return Value:
+//    Requested successor block
+//
+BasicBlock* BasicBlock::GetSucc(unsigned i, Compiler* comp)
+{
+    return GetSuccEdge(i, comp)->getDestinationBlock();
+}
+
 void BasicBlock::InitVarSets(Compiler* comp)
 {
     VarSetOps::AssignNoCopy(comp, bbVarUse, VarSetOps::MakeEmpty(comp));