MIPS: Move Maps' back pointers from "transitions" to "constructor" field

src/mips64/full-codegen-mips64.cc

Index: src/mips64/full-codegen-mips64.cc
diff --git a/src/mips64/full-codegen-mips64.cc b/src/mips64/full-codegen-mips64.cc
index 5fe0c5a6ee9f8e349761b3b0a9e5c2e78a8d6ef8..9809514d1477c83865c086c3e32aa03dba60ec93 100644
--- a/src/mips64/full-codegen-mips64.cc
+++ b/src/mips64/full-codegen-mips64.cc
@@ -3758,9 +3758,10 @@ void FullCodeGenerator::EmitClassOf(CallRuntime* expr) {
   STATIC_ASSERT(LAST_NONCALLABLE_SPEC_OBJECT_TYPE == LAST_TYPE - 1);
 
   // Check if the constructor in the map is a JS function.
-  __ ld(v0, FieldMemOperand(v0, Map::kConstructorOffset));
-  __ GetObjectType(v0, a1, a1);
-  __ Branch(&non_function_constructor, ne, a1, Operand(JS_FUNCTION_TYPE));
+  Register instance_type = a2;
+  __ GetMapConstructor(v0, v0, a1, instance_type);
+  __ Branch(&non_function_constructor, ne, instance_type,
+            Operand(JS_FUNCTION_TYPE));
 
   // v0 now contains the constructor function. Grab the
   // instance class name from there.