JIT: defer creating throw helper blocks (#93371)

Don't create any throw helper blocks until just before the late flow graph optimizations. 
This removes some of the flow graph updates from morph, with the aim of making it easier
to show that an RPO traversal in morph is safe.

Unfortunately it's hard to get these blocks in the exact same places
they were before, so there are some diffs.

src/coreclr/jit/compiler.cpp

@@ -5038,6 +5038,10 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
     // Insert GC Polls
     DoPhase(this, PHASE_INSERT_GC_POLLS, &Compiler::fgInsertGCPolls);
 
+    // Create any throw helper blocks that might be needed
+    //
+    DoPhase(this, PHASE_CREATE_THROW_HELPERS, &Compiler::fgCreateThrowHelperBlocks);
+
     if (opts.OptimizationEnabled())
     {
         // Optimize boolean conditions

src/coreclr/jit/compiler.h

@@ -6119,12 +6119,11 @@ class Compiler
     static unsigned acdHelper(SpecialCodeKind codeKind);
 
     AddCodeDsc* fgAddCodeList;
-    bool        fgAddCodeModf;
+    bool        fgRngChkThrowAdded;
     AddCodeDsc* fgExcptnTargetCache[SCK_COUNT];
 
-    BasicBlock* fgRngChkTarget(BasicBlock* block, SpecialCodeKind kind);
-
-    BasicBlock* fgAddCodeRef(BasicBlock* srcBlk, unsigned refData, SpecialCodeKind kind);
+    void fgAddCodeRef(BasicBlock* srcBlk, SpecialCodeKind kind);
+    PhaseStatus fgCreateThrowHelperBlocks();
 
 public:
     AddCodeDsc* fgFindExcptnTarget(SpecialCodeKind kind, unsigned refData);
@@ -6137,8 +6136,6 @@ class Compiler
     }
 
 private:
-    bool fgIsCodeAdded();
-
     bool fgIsThrowHlpBlk(BasicBlock* block);
 
 #if !FEATURE_FIXED_OUT_ARGS

src/coreclr/jit/compiler.hpp

@@ -3069,7 +3069,7 @@ inline Compiler::fgWalkResult Compiler::fgWalkTree(GenTree**    pTree,
 
 inline bool Compiler::fgIsThrowHlpBlk(BasicBlock* block)
 {
-    if (!fgIsCodeAdded())
+    if (!fgRngChkThrowAdded)
     {
         return false;
     }
@@ -3157,16 +3157,6 @@ inline unsigned Compiler::fgThrowHlpBlkStkLevel(BasicBlock* block)
 
 #endif // !FEATURE_FIXED_OUT_ARGS
 
-/*****************************************************************************
- *
- *  Return true if we've added any new basic blocks.
- */
-
-inline bool Compiler::fgIsCodeAdded()
-{
-    return fgAddCodeModf;
-}
-
 /*****************************************************************************
   Is the offset too big?
 */

src/coreclr/jit/compphases.h

@@ -99,6 +99,7 @@ CompPhaseNameMacro(PHASE_EXPAND_RTLOOKUPS,           "Expand runtime lookups",
 CompPhaseNameMacro(PHASE_EXPAND_STATIC_INIT,         "Expand static init",             false, -1, true)
 CompPhaseNameMacro(PHASE_EXPAND_TLS,                 "Expand TLS access",              false, -1, true)
 CompPhaseNameMacro(PHASE_INSERT_GC_POLLS,            "Insert GC Polls",                false, -1, true)
+CompPhaseNameMacro(PHASE_CREATE_THROW_HELPERS,       "Create throw helper blocks",     false, -1, true)
 CompPhaseNameMacro(PHASE_DETERMINE_FIRST_COLD_BLOCK, "Determine first cold block",     false, -1, true)
 CompPhaseNameMacro(PHASE_RATIONALIZE,                "Rationalize IR",                 false, -1, false)
 CompPhaseNameMacro(PHASE_SIMPLE_LOWERING,            "Do 'simple' lowering",           false, -1, false)

src/coreclr/jit/fgbasic.cpp

@@ -96,7 +96,6 @@ void Compiler::fgInit()
     // as the code that raises an exception when an array range check fails.
 
     fgAddCodeList = nullptr;
-    fgAddCodeModf = false;
 
     for (int i = 0; i < SCK_COUNT; i++)
     {
@@ -109,6 +108,9 @@ void Compiler::fgInit()
     /* This global flag is set whenever we remove a statement */
     fgStmtRemoved = false;
 
+    // This global flag is set when we create throw helper blocks
+    fgRngChkThrowAdded = false;
+
     /* Keep track of whether or not EH statements have been optimized */
     fgOptimizedFinally = false;