Implement floating point compare for mips fast-isel

Summary: Expand SelectCmp to handle floating point compare

Test Plan:
fpcmpa.ll
run 4 flavors of test-suite, mips32 r1/r2 O0/O2

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits, rfuhler

Differential Revision: http://reviews.llvm.org/D5567

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219530 91177308-0d34-0410-b5e6-96231b3b80d8
[mips] Actually mark that the default case is unreachable as this switch
is over a subset of condition codes.

This fixes the -Werror build which warns about use of uninitialized
variables in the default case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219531 91177308-0d34-0410-b5e6-96231b3b80d8