Fixing floating point register clobbering for MIPSr6 (32 and 64)
due to using of f31 floating point register as double compare register,
without saving the value of the register before using it.

TEST=cctest/test-debug/*
BUG=

Review URL: https://codereview.chromium.org/1346623002

Cr-Commit-Position: refs/heads/master@{#30765}