X87: Eliminate special keyed load string stub in favor of uniform handlers

X87: Eliminate special keyed load string stub in favor of uniform handlers

port r24661.

original commit message:

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

  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=

[chunyang.dai, 2014-10-17 04:44:11]

chunyang.dai@intel.com changed reviewers:
+ mvstanton@chromium.org, weiliang.lin@intel.com

[chunyang.dai, 2014-10-17 04:44:26]

WL.
PTAL.

[Weiliang, 2014-10-17 05:23:01]

lgtm

[Weiliang, 2014-10-17 05:24:59]

Committed as r24674