MIPS: Allow specifying base offset when constructing Keyed hydrogen instructions.

Port r21426 (17e6338)

This is preparation for pending hydrogen stub work that needs to access memory using KeyedLoad/KeyedStore operations where the base offset used for the accesses are is the the default (e.g. the size of an FixedArray header for FixedArrays or zero for external arrays).

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/296983003

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21443 ce2b1a6d-e550-0410-aec6-3dcde31c8c00