MIPS: Avoid HeapObject check in HStoreNamedField.

Port r21509 (78a54b0)

Original commit message:
This way an HStoreNamedField instruction can never deoptimize
itself, which is another important step towards a working
store elimination.

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/297143007

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21525 ce2b1a6d-e550-0410-aec6-3dcde31c8c00