Refactor code for finding x86 scratch register.
https://bugs.webkit.org/show_bug.cgi?id=122072

Patch by Nadav Rotem <nrotem@apple.com> on 2013-09-29
Reviewed by Geoffrey Garen.

* assembler/MacroAssemblerX86Common.h:
(JSC::MacroAssemblerX86Common::getUnusedRegister):
(JSC::MacroAssemblerX86Common::store8):
(JSC::MacroAssemblerX86Common::store16):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@156617 268f45cc-cd09-0410-ab3c-d52691b4dbfc