MIPS: [stubs] Further optimize Load/StoreGlobalViaContext stubs.

MIPS: [stubs] Further optimize Load/StoreGlobalViaContext stubs.

a) Don't use separate read/write slots for context globals, but share the
   same slot, which reduces the number of initial misses, and also saves
   some memory for large scripts.
b) No need to pass the name explicitly to the stubs; the runtime can
   extract the name from the ScopeInfo (the extension of the
   ScriptContext) on-demand easily without any performance impact.

BUG=

[paul.l..., 2015-07-26 18:55:31]

paul.lind@imgtec.com changed reviewers:
+ bmeurer@chromium.org, ishell@chromium.org, paul.lind@imgtec.com

[paul.l..., 2015-07-26 18:55:31]

l-g-t-m for inclusion in https://codereview.chromium.org/1250413002/

[Benedikt Meurer, 2015-07-27 10:45:53]

Thanks a lot.