Perf regression: changes in KeyedStoreIC introduced polymorphism.

Perf regression: changes in KeyedStoreIC introduced polymorphism.

When fixing bug 350884, I introduced an error that meant we went
polymorphic in KeyedStoreIC where we stayed monomorphic before. This
CL addresses the error, while preserving the bug fix for 350884.

R=verwaest@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=19893

[mvstanton, 2014-03-13 08:48:20]

Hi Toon,
There was an error in my KeyedStoreIC fix yesterday, happily found by Golem. The
problem was that too much of the logic for remaining monomorphic was moved under
the condition that we have a transitioning store mode. I also better protected
the else case. PTAL, thanks!
--Michael

[Toon Verwaest, 2014-03-13 10:47:00]

lgtm

[mvstanton, 2014-03-13 12:17:48]

Committed patchset #1 manually as r19893 (presubmit successful).