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

src/mips/lithium-codegen-mips.cc

Index: src/mips/lithium-codegen-mips.cc
diff --git a/src/mips/lithium-codegen-mips.cc b/src/mips/lithium-codegen-mips.cc
index 0244e7ee0d5a3391a50948ec1668f628ee844eed..3be283de831ad40240e83daafb6c1cf5044f8cc9 100644
--- a/src/mips/lithium-codegen-mips.cc
+++ b/src/mips/lithium-codegen-mips.cc
@@ -2657,14 +2657,16 @@ void LCodeGen::EmitClassOfTest(Label* is_true,
 
   // Now we are in the FIRST-LAST_NONCALLABLE_SPEC_OBJECT_TYPE range.
   // Check if the constructor in the map is a function.
-  __ lw(temp, FieldMemOperand(temp, Map::kConstructorOffset));
+  Register instance_type = scratch1();
+  DCHECK(!instance_type.is(temp));
+  DCHECK(!instance_type.is(temp2));
+  __ GetMapConstructor(temp, temp, temp2, instance_type);
 
   // Objects with a non-function constructor have class 'Object'.
-  __ GetObjectType(temp, temp2, temp2);
   if (String::Equals(class_name, isolate()->factory()->Object_string())) {
-    __ Branch(is_true, ne, temp2, Operand(JS_FUNCTION_TYPE));
+    __ Branch(is_true, ne, instance_type, Operand(JS_FUNCTION_TYPE));
   } else {
-    __ Branch(is_false, ne, temp2, Operand(JS_FUNCTION_TYPE));
+    __ Branch(is_false, ne, instance_type, Operand(JS_FUNCTION_TYPE));
   }
 
   // temp now contains the constructor function. Grab the