Support selecting Ubfx for shift-mask and mask-shift operations. Also, rename
the shifts to match the instruction names.

BUG=
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/633123002

Patch from Martyn Capewell <m.m.capewell@googlemail.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24482 ce2b1a6d-e550-0410-aec6-3dcde31c8c00