MIPS: Optimize NumberTagU

Port r18162 (8377a09b)

Original commit message:
Optimize register constaints for NumberTagU Lithium instruction, and adjust code generated to match.

BUG=
R=gergely@homejinni.com

Review URL: https://codereview.chromium.org/97083002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18165 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
MIPS: Fix --new-string-add feature.

This commit fixes the arguments of NewStringAdd stub in full-codegen.

BUG=
R=gergely@homejinni.com

Review URL: https://codereview.chromium.org/96593005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18166 ce2b1a6d-e550-0410-aec6-3dcde31c8c00