[arm] Recognize SXTB, SXTH, UXTB and UXTH.

TEST=cctest,msjunit/asm,unittests
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/709123005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25228 ce2b1a6d-e550-0410-aec6-3dcde31c8c00