MIPS: Truncate booleans to 0/1 in truncating t-to-i.

Port r17166 (0a4bf790)

Original commit message:
Truncate booleans to 0/1 in truncating t-to-i.

Thanks to weiliang.lin2@gmail.com for discovering the issue.

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/26742006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17173 ce2b1a6d-e550-0410-aec6-3dcde31c8c00