SimplifyAllocStack: bail if the address of the allocated existential escapes

This is not a problem if the alloc_stack already contains the opened existential.
Fixes a miscompile
rdar://148253142

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyAllocStack.swift

@@ -257,11 +257,15 @@ private extension AllocStackInst {
       case is CheckedCastAddrBranchInst, is UnconditionalCheckedCastAddrInst:
         // To construct a new cast instruction we need a formal type.
         requiresLegalFormalType = true
-        fallthrough
-      case is UncheckedAddrCastInst:
         if use != use.instruction.operands[0] {
           return nil
         }
+      case is UncheckedAddrCastInst:
+        if self.type.isExistential {
+          // Bail if the address of the original existential escapes.
+          // This is not a problem if the alloc_stack already contains the opened existential.
+          return nil
+        }
       default:
         return nil
       }

test/SILOptimizer/simplify_alloc_stack.sil

@@ -365,3 +365,19 @@ bb0(%0 : @owned $C2):
   dealloc_stack %1
   return %5
 }
+
+// CHECK-LABEL: sil [ossa] @dont_replace_existential_with_escaping_addr :
+// CHECK:         alloc_stack $any P
+// CHECK:       } // end sil function 'dont_replace_existential_with_escaping_addr'
+sil [ossa] @dont_replace_existential_with_escaping_addr : $@convention(thin) <V> (@thick V.Type, @owned C2) -> @out V {
+bb0(%0 : $*V, %1 : $@thick V.Type, %2 : @owned $C2):
+  %114 = alloc_stack $any P
+  %117 = init_existential_addr %114, $C2
+  store %2 to [init] %117
+  %129 = unchecked_addr_cast %114 to $*V
+  copy_addr %129 to [init] %0
+  destroy_addr %114
+  dealloc_stack %114
+  %137 = tuple ()
+  return %137
+}