[vm/compiler] Don't drop null initialization on non-Dart objects.

TEST=ffi/vmspecific_handle_test on vm-aot-optimization-level-linux-release-x64

Fixes: #55399
Cq-Include-Trybots: luci.dart.try:vm-aot-optimization-level-linux-release-x64-try
Change-Id: I90ef44d6bd678d56a3d183b78a51e80b475a2764
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/361601
Auto-Submit: Tess Strickland <sstrickl@google.com>
Commit-Queue: Daco Harkes <dacoharkes@google.com>
Reviewed-by: Daco Harkes <dacoharkes@google.com>

runtime/vm/compiler/backend/il.cc

@@ -1028,7 +1028,8 @@ Instruction* StoreFieldInstr::Canonicalize(FlowGraph* flow_graph) {
   // Context objects can be allocated uninitialized as a performance
   // optimization in JIT mode - however in AOT mode we always allocate them
   // null initialized.
-  if (is_initialization_ && slot().representation() == kTagged &&
+  if (is_initialization_ && !slot().has_untagged_instance() &&
+      slot().representation() == kTagged &&
       (!slot().IsContextSlot() ||
        !instance()->definition()->IsAllocateUninitializedContext()) &&
       value()->BindsToConstantNull()) {