JIT: prepare for instrumentation before incorporating profile counts

Otherwise the spanning tree we generate may be biased by the profile data
and not match the spanning tree we generated in Tier0.

Fixes dotnet#85799.

src/coreclr/jit/block.h

@@ -1089,15 +1089,13 @@ struct BasicBlock : private LIR::Range
     BlockSet bbReach; // Set of all blocks that can reach this one
 
     union {
-        BasicBlock* bbIDom;   // Represent the closest dominator to this block (called the Immediate
-                              // Dominator) used to compute the dominance tree.
-        FlowEdge* bbLastPred; // Used early on by fgLinkBasicBlock/fgAddRefPred
+        BasicBlock* bbIDom;          // Represent the closest dominator to this block (called the Immediate
+                                     // Dominator) used to compute the dominance tree.
+        FlowEdge* bbLastPred;        // Used early on by fgLinkBasicBlock/fgAddRefPred
+        void*     bbSparseProbeList; // Used early on by fgInstrument
     };
 
-    union {
-        void* bbSparseCountInfo; // Used early on by fgIncorporateEdgeCounts
-        void* bbSparseProbeList; // Used early on by fgInstrument
-    };
+    void* bbSparseCountInfo; // Used early on by fgIncorporateEdgeCounts
 
     unsigned bbPreorderNum;  // the block's  preorder number in the graph (1...fgMaxBBNum]
     unsigned bbPostorderNum; // the block's postorder number in the graph (1...fgMaxBBNum]

src/coreclr/jit/compiler.cpp

@@ -4496,6 +4496,14 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
     };
     DoPhase(this, PHASE_PRE_IMPORT, preImportPhase);
 
+    // If we're going to instrument code, we may need to prepare before
+    // we import. Also do this before we read in any profile data.
+    //
+    if (compileFlags->IsSet(JitFlags::JIT_FLAG_BBINSTR))
+    {
+        DoPhase(this, PHASE_IBCPREP, &Compiler::fgPrepareToInstrumentMethod);
+    }
+
     // Incorporate profile data.
     //
     // Note: the importer is sensitive to block weights, so this has
@@ -4505,14 +4513,6 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
     DoPhase(this, PHASE_INCPROFILE, &Compiler::fgIncorporateProfileData);
     activePhaseChecks &= ~PhaseChecks::CHECK_PROFILE;
 
-    // If we're going to instrument code, we may need to prepare before
-    // we import.
-    //
-    if (compileFlags->IsSet(JitFlags::JIT_FLAG_BBINSTR))
-    {
-        DoPhase(this, PHASE_IBCPREP, &Compiler::fgPrepareToInstrumentMethod);
-    }
-
     // If we are doing OSR, update flow to initially reach the appropriate IL offset.
     //
     if (opts.IsOSR())

src/coreclr/jit/fgprofile.cpp

@@ -1220,6 +1220,16 @@ void Compiler::WalkSpanningTree(SpanningTreeVisitor* visitor)
             break;
         }
     }
+
+    // Notify visitor of remaining blocks
+    //
+    for (BasicBlock* const block : Blocks())
+    {
+        if (!BlockSetOps::IsMember(this, marked, block->bbNum))
+        {
+            visitor->VisitBlock(block);
+        }
+    }
 }
 
 // Map a block into its schema key we will use for storing basic blocks.