Add new HSeqStringGetChar instruction.

This instruction is required for copying characters from sequential
strings in the hydrogenized StringAddStub.

BUG=v8:2990
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/63863005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17565 ce2b1a6d-e550-0410-aec6-3dcde31c8c00