[runtime] Delete extra variables

No need to reassign those JSTypedArrays.

Bug: v8:6704
Change-Id: Ide1f8bb119285b57ea85b8e710358c917244f801
Reviewed-on: https://chromium-review.googlesource.com/641474
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47693}

src/runtime/runtime-typedarray.cc

@@ -240,12 +240,10 @@ RUNTIME_FUNCTION(Runtime_TypedArraySetFastCases) {
     return Smi::FromInt(TYPED_ARRAY_SET_NON_TYPED_ARRAY);
   }
 
-  CONVERT_ARG_HANDLE_CHECKED(JSTypedArray, target_obj, 0);
-  CONVERT_ARG_HANDLE_CHECKED(JSTypedArray, source_obj, 1);
+  CONVERT_ARG_HANDLE_CHECKED(JSTypedArray, target, 0);
+  CONVERT_ARG_HANDLE_CHECKED(JSTypedArray, source, 1);
   CONVERT_NUMBER_ARG_HANDLE_CHECKED(offset_obj, 2);
 
-  Handle<JSTypedArray> target(JSTypedArray::cast(*target_obj));
-  Handle<JSTypedArray> source(JSTypedArray::cast(*source_obj));
   size_t offset = 0;
   CHECK(TryNumberToSize(*offset_obj, &offset));
   size_t target_length = target->length_value();