[SubZero] Fix floating-point comparison for MIPS

The patch fixes code generation and encoding of floating-point comparison.
All floating-point comparison related test in test_fcmp cross test pass (after removing vector related tests):
TotalTests=123904 Passes=123904 Failures=0

R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/2357143002 .

Patch from Jaydeep Patil <jaydeep.patil@imgtec.com>.
7 files changed