Allow more virtual registers to be encoded in LUnallocated.

This is a preparation which allows us to bump the virtual register width
from 15 to 18 bit without sacrificing width for other fields inside an
unallocated lithium operand.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/14639008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14513 ce2b1a6d-e550-0410-aec6-3dcde31c8c00