MIPS: Fix AccessArgumentsAt on ARM. Use register for index.

Port r14885 (63dd99b)

BUG=

Review URL: https://codereview.chromium.org/16215002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14892 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/mips/lithium-mips.cc

@@ -2360,7 +2360,7 @@ LInstruction* LChunkBuilder::DoAccessArgumentsAt(HAccessArgumentsAt* instr) {
     index = UseOrConstant(instr->index());
   } else {
     length = UseTempRegister(instr->length());
-    index = Use(instr->index());
+    index = UseRegisterAtStart(instr->index());
   }
   return DefineAsRegister(new(zone()) LAccessArgumentsAt(args, length, index));
 }