MIPS: Add support to load/store byte fields.
MIPS: Add support to load/store byte fields.

Port r17079 (0885ac2)

Original commit message:
This adds a new Byte representation and support for zero-extended
loads in HLoadNamedField and truncated stores in HStoreNamedField.

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/25460003

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: b158db1e4b4ab85d4c9e510fdef4b1e8c614b15b@17085 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh