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

BUG=

Review URL: https://codereview.chromium.org/950523003

Cr-Commit-Position: refs/heads/master@{#26860}

src/mips/lithium-codegen-mips.cc

@@ -2659,7 +2659,6 @@ void LCodeGen::EmitClassOfTest(Label* is_true,
   // Check if the constructor in the map is a function.
   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'.

src/mips/macro-assembler-mips.cc

@@ -4387,7 +4387,7 @@ void MacroAssembler::TryGetFunctionPrototype(Register function,
     // Non-instance prototype: Fetch prototype from constructor field
     // in initial map.
     bind(&non_instance);
-    GetMapConstructor(result, result, scratch, at);
+    GetMapConstructor(result, result, scratch, scratch);
   }
 
   // All done.

src/mips64/lithium-codegen-mips64.cc

@@ -2620,7 +2620,6 @@ void LCodeGen::EmitClassOfTest(Label* is_true,
   // Check if the constructor in the map is a function.
   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'.

src/mips64/macro-assembler-mips64.cc

@@ -4309,7 +4309,7 @@ void MacroAssembler::TryGetFunctionPrototype(Register function,
     // Non-instance prototype: Fetch prototype from constructor field
     // in initial map.
     bind(&non_instance);
-    GetMapConstructor(result, result, scratch, at);
+    GetMapConstructor(result, result, scratch, scratch);
   }
 
   // All done.