[mips][FastISel] Implement the select statement for MIPS FastISel.

Summary: Implement the LLVM IR select statement for MIPS FastISelsel.

Based on a patch by Reed Kotler.

Test Plan:
"Make check" test included now.
Passes test-suite at O2/O0 mips32 r1/r2.

Reviewers: dsanders, rkotler

Subscribers: llvm-commits, rfuhler

Differential Revision: http://reviews.llvm.org/D6774

llvm-svn: 238756