MIPS: Change CmpWeakValue to a more MIPS like GetWeakValue.

MIPS: Change CmpWeakValue to a more MIPS like GetWeakValue.

This approach saves one instruction by eliminating the use of Subu as a
pseudo-comparison.

BUG=

Review URL: https://codereview.chromium.org/779793004

Cr-Commit-Position: refs/heads/master@{#25662}