SILGen: use builtin prepareInitialization instead of zeroInitializer when generating a value constructor for an empty non-copyable type

`zeroInitializer` is blocking optimizations. It is not needed because the constructor will initialize the memory anyway.
We only need to tell mandatory passes that this memory should be treated as initialized.

lib/SILGen/SILGenConstructor.cpp

@@ -738,8 +738,13 @@ void SILGenFunction::emitValueConstructor(ConstructorDecl *ctor) {
     // move only structs are non-trivial, so we need to handle this here.
     if (nominal->getAttrs().hasAttribute<RawLayoutAttr>()) {
       // Raw memory is not directly decomposable, but we still want to mark
-      // it as initialized. Use a zero initializer.
-      B.createZeroInitAddr(ctor, selfLV.getLValueAddress());
+      // it as initialized.
+      auto prepInit = getBuiltinValueDecl(getASTContext(),
+                getASTContext().getIdentifier("prepareInitialization"));
+      B.createBuiltin(ctor, prepInit->getBaseIdentifier(),
+                           SILType::getEmptyTupleType(getASTContext()),
+                           SubstitutionMap(),
+                           selfLV.getLValueAddress());
     } else if (isa<StructDecl>(nominal)
                && lowering.getLoweredType().isMoveOnly()
                && nominal->getStoredProperties().empty()) {

test/SILOptimizer/moveonly_raw_layout.swift

@@ -20,7 +20,7 @@ struct Lock: ~Copyable {
     // CHECK-NEXT: sil{{.*}} @[[INIT:\$.*4LockV.*fC]] :
     init() {
         // CHECK-NOT: destroy_addr
-        // CHECK: builtin "zeroInitializer"({{%.*}} : $*Lock)
+        // CHECK: builtin "prepareInitialization"({{%.*}} : $*Lock)
         // CHECK-NOT: destroy_addr
         // CHECK: [[F:%.*]] = function_ref @init_lock
         // CHECK: apply [[F]](

test/SILOptimizer/stdlib/Cell.swift

@@ -20,7 +20,7 @@ public struct Cell<T: ~Copyable>: ~Copyable {
 
   // CHECK-LABEL: sil {{.*}} @$s4CellAAVAARi_zrlEyAByxGxcfC : $@convention(method) <T where T : ~Copyable> (@in T, @thin Cell<T>.Type) -> @out Cell<T> {
   // CHECK:       bb0({{%.*}} : $*Cell<T>, [[VALUE:%.*]] : $*T, {{%.*}} : $@thin Cell<T>.Type):
-  // CHECK:         {{%.*}} = builtin "zeroInitializer"([[SELF:%.*]] : $*Cell<T>) : $()
+  // CHECK:         {{%.*}} = builtin "prepareInitialization"([[SELF:%.*]] : $*Cell<T>) : $()
   // CHECK-NEXT:    [[RAW_LAYOUT_ADDR:%.*]] = builtin "addressOfRawLayout"<Cell<T>>([[SELF]] : $*Cell<T>) : $Builtin.RawPointer
   // CHECK-NEXT:    [[POINTER:%.*]] = struct $UnsafeMutablePointer<T> ([[RAW_LAYOUT_ADDR]] : $Builtin.RawPointer)
   //                Calling 'UnsafeMutablePointer<T>.initialize(to:)'