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=
R=weiliang.lin@intel.com

Review URL: https://codereview.chromium.org/643783005

Patch from Chunyang Dai <chunyang.dai@intel.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24674 ce2b1a6d-e550-0410-aec6-3dcde31c8c00