MIPS: Add Smi support to Shl

Port r15910 (8e65c434)

BUG=

Review URL: https://codereview.chromium.org/20808004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15913 ce2b1a6d-e550-0410-aec6-3dcde31c8c00