MIPS64: ReceiverCheckMode needs to be utilized further.

Port r24704 (f43a27ee)

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

BUG=
R=paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/645823003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24707 ce2b1a6d-e550-0410-aec6-3dcde31c8c00