JIT: Delay creation of scratch block needed by backend

src/coreclr/jit/flowgraph.cpp

@@ -2307,15 +2307,6 @@ PhaseStatus Compiler::fgAddInternal()
     // type with a runtime lookup
     madeChanges |= fgCreateFiltersForGenericExceptions();
 
-    // The backend requires a scratch BB into which it can safely insert a P/Invoke method prolog if one is
-    // required. Similarly, we need a scratch BB for poisoning and when we have Swift parameters to reassemble.
-    // Create it here.
-    if (compMethodRequiresPInvokeFrame() || compShouldPoisonFrame() || lvaHasAnySwiftStackParamToReassemble())
-    {
-        madeChanges |= fgEnsureFirstBBisScratch();
-        fgFirstBB->SetFlags(BBF_DONT_REMOVE);
-    }
-
     /*
     <BUGNUM> VSW441487 </BUGNUM>
 

src/coreclr/jit/lower.cpp

@@ -7812,6 +7812,18 @@ void Lowering::WidenSIMD12IfNecessary(GenTreeLclVarCommon* node)
 
 PhaseStatus Lowering::DoPhase()
 {
+    // We need a scratch BB into which it can safely insert a P/Invoke method
+    // prolog if one is required. Similarly, we need a scratch BB for poisoning
+    // and when we have Swift parameters to reassemble. Create it here.
+    if (comp->compMethodRequiresPInvokeFrame() || comp->compShouldPoisonFrame() ||
+        comp->lvaHasAnySwiftStackParamToReassemble())
+    {
+        if (comp->fgEnsureFirstBBisScratch())
+        {
+            comp->fgInvalidateDfsTree();
+        }
+    }
+
     // If we have any PInvoke calls, insert the one-time prolog code. We'll insert the epilog code in the
     // appropriate spots later. NOTE: there is a minor optimization opportunity here, as we still create p/invoke
     // data structures and setup/teardown even if we've eliminated all p/invoke calls due to dead code elimination.