[turbofan] Eliminate redundant masking operations for word8/word16 stores.

There's no need to apply 0xff when storing 8-bit values or 0xffff when
storing 16-bit values.

TEST=unittests
R=titzer@chromium.org

Review URL: https://codereview.chromium.org/642033002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24492 ce2b1a6d-e550-0410-aec6-3dcde31c8c00