[mips][FastISel] Implement bswap.

Summary: Implement bswap intrinsic for MIPS FastISel. It's very different for misp32 r1/r2 .

Based on a patch by Reed Kotler.

Test Plan:
bswap1.ll
test-suite

Reviewers: dsanders, rkotler

Subscribers: llvm-commits, rfuhler

Differential Revision: http://reviews.llvm.org/D7219

llvm-svn: 238760