Always invalidate DFS tree after layout

src/coreclr/jit/compiler.cpp

@@ -5227,6 +5227,13 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
             DoPhase(this, PHASE_OPTIMIZE_LAYOUT, &Compiler::fgSearchImprovedLayout);
         }
 
+        // 3-opt will mess with post-order numbers regardless of whether it modifies anything,
+        // so we always need to invalidate the flowgraph annotations after.
+        // If we didn't run 3-opt, we might still have a profile-aware DFS tree computed during LSRA available.
+        // This tree's presence can trigger asserts if pre/postorder numbers are recomputed,
+        // so invalidate the tree in both cases.
+        fgInvalidateDfsTree();
+
         // Now that the flowgraph is finalized, run post-layout optimizations.
         //
         DoPhase(this, PHASE_OPTIMIZE_POST_LAYOUT, &Compiler::optOptimizePostLayout);

src/coreclr/jit/fgopt.cpp

@@ -5295,9 +5295,6 @@ PhaseStatus Compiler::fgSearchImprovedLayout()
         JITDUMP("No hot blocks found. Skipping reordering.\n");
     }
 
-    // 3-opt will mess with post-order numbers regardless of whether it modifies anything,
-    // so we always need to invalidate the flowgraph annotations after.
-    fgInvalidateDfsTree();
     return modified ? PhaseStatus::MODIFIED_EVERYTHING : PhaseStatus::MODIFIED_NOTHING;
 }