MIPS: Eliminate special keyed load string stub in favor of uniform handlers.

MIPS: Eliminate special keyed load string stub in favor of uniform handlers.

Port r24661 (5499efb)

Original commit message:
KeyedLoadIC installs a special case if the receiver is a string.
Although there are several maps for strings, in practice we seem to
be able to treat them individually because a given KeyedLoad site
only sees 1-2 string types.

BUG=
R=paul.lind@imgtec.com

Committed: https://code.google.com/p/v8/source/detail?r=24668

[balazs.kilvady, 2014-10-16 16:29:09]

balazs.kilvady@imgtec.com changed reviewers:
+ akos.palfi@imgtec.com, danno@chromium.org, dusan.milosavljevic@imgtec.com,
gergely.kis@imgtec.com, mvstanton@chromium.org, paul.lind@imgtec.com

[paul.l..., 2014-10-16 16:37:57]

lgtm

[balazs.kilvady, 2014-10-16 16:39:58]

Committed patchset #1 (id:1) manually as 24668 (presubmit successful).