MIPS: Optimize Integer32ToSmi.

Port r18084 (426bc19)

Original commit message:
Optimize register constraints and code generated for Integer32ToSmi Lithium
instruction.

TEST=none
BUG=
R=gergely@homejinni.com

Review URL: https://codereview.chromium.org/88623004

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18089 ce2b1a6d-e550-0410-aec6-3dcde31c8c00