MIPS: Lithium: avoid registers for constants when possible.

Port r14460 (9e3dab0c)

BUG=

Review URL: https://codereview.chromium.org/14520016

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14473 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
MIPS: Track storage types of instance variables.

Port r14464 (70300e97) and r14470 (65af80fc)

BUG=

Review URL: https://codereview.chromium.org/14305024

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14474 ce2b1a6d-e550-0410-aec6-3dcde31c8c00