Allow more virtual registers to be encoded in LUnallocated.

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

Committed: http://code.google.com/p/v8/source/detail?r=14513

[Sven Panne, 2013-05-02 08:40:17]

lgtm

[Michael Starzinger, 2013-05-02 09:51:16]

Committed patchset #1 manually as r14513 (presubmit successful).