[mips] Fix assertion on i128 addition/subtraction on MIPS64

Summary:
In addition to the included tests, this fixes
test/CodeGen/Generic/i128-addsub.ll on a mips64 host.

Reviewers: atanasyan, sagar, vmedic

Reviewed By: vmedic

Subscribers: sdkie, llvm-commits

Differential Revision: http://reviews.llvm.org/D6610

llvm-svn: 227003