X87:  ReceiverCheckMode needs to be utilized further.

port r24704.

original commit message:

  ReceiverCheckMode needs to be utilized further.

  The parameter wasn't being passed appropriately, and there was an extra
   opportunity to use mode RECEIVER_IS_STRING in SubStringStub.

BUG=
R=weiliang.lin@intel.com

Review URL: https://codereview.chromium.org/664973002

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24714 ce2b1a6d-e550-0410-aec6-3dcde31c8c00