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

src/mips/full-codegen-mips.cc

Index: src/mips/full-codegen-mips.cc
diff --git a/src/mips/full-codegen-mips.cc b/src/mips/full-codegen-mips.cc
index c5749fcf3a6d472e83df5dc93e5403223c824698..91753162faf4e1ecc7d30af4f0e8ae583eda6091 100644
--- a/src/mips/full-codegen-mips.cc
+++ b/src/mips/full-codegen-mips.cc
@@ -3757,9 +3757,10 @@ 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.
-  __ lw(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.