port r21422 (38edcd2)

Original commit message:
On ia32, SAHF is guaranteed. It's only necessary to probe for it on x64.

BUG=
R=danno@chromium.org

Review URL: https://codereview.chromium.org/301083002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21602 ce2b1a6d-e550-0410-aec6-3dcde31c8c00