[ic] Optimize length access in AccessorAssembler::ExtendPropertiesBackingStore.

When the optimal parameter mode is untagged (i.e. on x64), it's more
efficient to read the length of the properties backing store untagged
instead of reading the tagged value first and then untagging it
explicitly.

Change-Id: I0f685523e11da12fe51fad138bf972ee743c3ac8
Reviewed-on: https://chromium-review.googlesource.com/461138Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44169}

src/ic/accessor-assembler.cc

@@ -1055,11 +1055,12 @@ Node* AccessorAssembler::PrepareValueForStore(Node* handler_word, Node* holder,
 }
 
 void AccessorAssembler::ExtendPropertiesBackingStore(Node* object) {
-  Node* properties = LoadProperties(object);
-  Node* length = LoadFixedArrayBaseLength(properties);
-
   ParameterMode mode = OptimalParameterMode();
-  length = TaggedToParameter(length, mode);
+
+  Node* properties = LoadProperties(object);
+  Node* length = (mode == INTPTR_PARAMETERS)
+                     ? LoadAndUntagFixedArrayBaseLength(properties)
+                     : LoadFixedArrayBaseLength(properties);
 
   Node* delta = IntPtrOrSmiConstant(JSObject::kFieldsAdded, mode);
   Node* new_capacity = IntPtrOrSmiAdd(length, delta, mode);