DeadObjectElimination: handle the builtin "prepareInitialization"

which allows e.g. eliminating dead `Atomic` values

Author: eeckstein

lib/SILOptimizer/Transforms/DeadObjectElimination.cpp

@@ -318,7 +318,8 @@ static bool canZapInstruction(SILInstruction *Inst, bool acceptRefCountInsts,
   // The value form of zero init is not a user of any operand. The address
   // form however is easily zappable because it's always a trivial store.
   if (auto bi = dyn_cast<BuiltinInst>(Inst)) {
-    if (bi->getBuiltinKind() == BuiltinValueKind::ZeroInitializer) {
+    if (bi->getBuiltinKind() == BuiltinValueKind::ZeroInitializer ||
+        bi->getBuiltinKind() == BuiltinValueKind::PrepareInitialization) {
       return true;
     }
   }

test/SILOptimizer/dead_alloc_elim.sil

@@ -721,3 +721,15 @@ bb0:
   %1 = tuple ()
   return %1 : $()
 }
+
+// CHECK-LABEL: @dead_prepare_init
+// CHECK-NOT: alloc_stack
+// // CHECK: } // end sil function 'dead_prepare_init'
+sil @dead_prepare_init : $@convention(thin) () -> () {
+bb0:
+  %0 = alloc_stack $Kl
+  %empty = builtin "prepareInitialization"(%0 : $*Kl) : $()
+  dealloc_stack %0 : $*Kl
+  %1 = tuple ()
+  return %1 : $()
+}