MIPS: Fix bug in StringConstructCode builtin.

src/mips/builtins-mips.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 255 matching lines @@
   __ mov(argument, a0);
   __ IncrementCounter(counters->string_ctor_conversions(), 1, a3, t0);
   __ Branch(&argument_is_string);
 
   // Invoke the conversion builtin and put the result into a2.
   __ bind(&convert_argument);
   __ push(function);  // Preserve the function.
   __ IncrementCounter(counters->string_ctor_conversions(), 1, a3, t0);
   {
     FrameScope scope(masm, StackFrame::INTERNAL);
-    __ push(v0);
+    __ push(a0);
     __ InvokeBuiltin(Builtins::TO_STRING, CALL_FUNCTION);
   }
   __ pop(function);
   __ mov(argument, v0);
   __ Branch(&argument_is_string);
 
   // Load the empty string into a2, remove the receiver from the
   // stack, and jump back to the case where the argument is a string.
   __ bind(&no_arguments);
   __ LoadRoot(argument, Heap::kempty_stringRootIndex);
@@ skipping 1229 matching lines @@
   __ bind(&dont_adapt_arguments);
   __ Jump(a3);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_MIPS