MIPS: Combine %_SubString and %_StringCharAt.

Port r13700 (587dc7c1)

BUG=

Review URL: https://codereview.chromium.org/12310087

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13739 ce2b1a6d-e550-0410-aec6-3dcde31c8c00