[wasm] Cleanup unneeded casts to WasmInstanceObject

src/frames.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/frames.h"
 
 #include <memory>
 #include <sstream>
 
 #include "src/base/bits.h"
@@ skipping 1541 matching lines @@
   return static_cast<Code*>(isolate()->FindCodeObject(pc()));
 }
 
 void WasmFrame::Iterate(ObjectVisitor* v) const { IterateCompiledFrame(v); }
 
 Address WasmFrame::GetCallerStackPointer() const {
   return fp() + ExitFrameConstants::kCallerSPOffset;
 }
 
 WasmInstanceObject* WasmFrame::wasm_instance() const {
-  Object* ret = wasm::GetOwningWasmInstance(LookupCode());
-  // This is a live stack frame, there must be a live wasm instance available.
-  DCHECK_NOT_NULL(ret);
-  return WasmInstanceObject::cast(ret);
+  WasmInstanceObject* obj = wasm::GetOwningWasmInstance(LookupCode());
+  // This is a live stack frame; it must have a live instance.
+  DCHECK_NOT_NULL(obj);
+  return obj;
 }
 
 uint32_t WasmFrame::function_index() const {
   FixedArray* deopt_data = LookupCode()->deoptimization_data();
   DCHECK(deopt_data->length() == 2);
   return Smi::cast(deopt_data->get(1))->value();
 }
 
 Script* WasmFrame::script() const {
   Handle<JSObject> instance(JSObject::cast(wasm_instance()), isolate());
@@ skipping 467 matching lines @@
   for (StackFrameIterator it(isolate); !it.done(); it.Advance()) {
     StackFrame* frame = AllocateFrameCopy(it.frame(), zone);
     list.Add(frame, zone);
   }
   return list.ToVector();
 }
 
 
 }  // namespace internal
 }  // namespace v8