We never use the smi register in places where it matters that r12 as
a base register needs an extra SIB byte. Might as well free up the
unencumbered r15 register instead.

Review URL: http://codereview.chromium.org/6647015

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7113 ce2b1a6d-e550-0410-aec6-3dcde31c8c00