JIT: run remorph stress before fgRemoveDeadBlocks (#68420)

Since the former may want to consult reachability sets and the latter may
render them invalid.

Also move the point at which we globally disable dominators up since they
are also no longer valid after `fgRemoveDeadBlocks`.

Fixes #68306.
Fixes #67871.

src/coreclr/jit/compiler.cpp

@@ -4972,9 +4972,17 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
         }
     }
 
+#ifdef DEBUG
+    // Run this before we potentially tear down dominators.
+    fgDebugCheckLinks(compStressCompile(STRESS_REMORPH_TREES, 50));
+#endif
+
     // Remove dead blocks
     DoPhase(this, PHASE_REMOVE_DEAD_BLOCKS, &Compiler::fgRemoveDeadBlocks);
 
+    // Dominator and reachability sets are no longer valid.
+    fgDomsComputed = false;
+
     // Insert GC Polls
     DoPhase(this, PHASE_INSERT_GC_POLLS, &Compiler::fgInsertGCPolls);
 
@@ -4983,8 +4991,6 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
     DoPhase(this, PHASE_DETERMINE_FIRST_COLD_BLOCK, &Compiler::fgDetermineFirstColdBlock);
 
 #ifdef DEBUG
-    fgDebugCheckLinks(compStressCompile(STRESS_REMORPH_TREES, 50));
-
     // Stash the current estimate of the function's size if necessary.
     if (verbose)
     {
@@ -5026,12 +5032,6 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
 
     // Assign registers to variables, etc.
 
-    ///////////////////////////////////////////////////////////////////////////////
-    // Dominator and reachability sets are no longer valid. They haven't been
-    // maintained up to here, and shouldn't be used (unless recomputed).
-    ///////////////////////////////////////////////////////////////////////////////
-    fgDomsComputed = false;
-
     // Create LinearScan before Lowering, so that Lowering can call LinearScan methods
     // for determining whether locals are register candidates and (for xarch) whether
     // a node is a containable memory op.