[mips] Correct 128-bit shifts on 64-bit targets.

Summary:
The existing code was correct for 32-bit GPR's but not 64-bit GPR's. It now
accounts for both cases.

Reviewers: vkalintiris

Reviewed By: vkalintiris

Subscribers: llvm-commits, mohit.bhakkad, sagar

Differential Revision: http://reviews.llvm.org/D9337

llvm-svn: 236099