SILMem2Reg: fix a problem with leaking enum values

When optimizing an enum `store` to an `alloc_stack`, require that all uses are in the same block.
Otherwise it could be a `switch_enum` of an optional where the none-case does not have a destroy of the enum value.
After transforming such an `alloc_stack`, the value would leak in the none-case block.

It fixes the same OSSA verification error as done for TempRValueOpt in a previous commit.

lib/SILOptimizer/Transforms/SILMem2Reg.cpp

@@ -1757,6 +1757,14 @@ bool MemoryToRegisters::promoteSingleAllocation(AllocStackInst *alloc) {
       b.createDeallocStack(next->getLoc(), alloc);
     }
     return true;
+  } else {
+    // For enums we require that all uses are in the same block.
+    // Otherwise there could be a switch_enum of an optional where the none-case
+    // does not have a destroy of the enum value.
+    // After transforming such an alloc_stack, the value would leak in the none-
+    // case block.
+    if (f.hasOwnership() && alloc->getType().isOrHasEnum())
+      return false;
   }
 
   LLVM_DEBUG(llvm::dbgs() << "*** Need to insert BB arguments for " << *alloc);