[sil-stack-promotion] Don't try to promote an already promoted allocation.

It just saves some compile time. NFC.

Author: swiftix

lib/SILOptimizer/Transforms/StackPromotion.cpp

@@ -216,7 +216,7 @@ class StackPromoter {
 static bool isPromotableAllocInst(SILInstruction *I) {
   // Check for swift object allocation.
   if (auto *ARI = dyn_cast<AllocRefInst>(I)) {
-    if (!ARI->isObjC())
+    if (!ARI->isObjC() && !ARI->canAllocOnStack())
       return true;
     return false;
   }