MIPS: Add new HSeqStringGetChar instruction.

Port r17565 (dce7927c)

Original commit message:
This instruction is required for copying characters from sequential
strings in the hydrogenized StringAddStub.

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/65483002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17574 ce2b1a6d-e550-0410-aec6-3dcde31c8c00