MIPS: Fix after 'Reland remaining parts of 'Use weak cells in map checks in polymorphic ICs''.

Fix d2e54925caa8b14988a46a912a8b061bf4c6cbf3

In 'MIPS: Change CmpWeakValue to a more MIPS like GetWeakValue.'
a25003cfa6eac88635c12b51ec6ad74fed0d91a1
we switched to use GetWeakValue.

BUG=

Review URL: https://codereview.chromium.org/782273004

Cr-Commit-Position: refs/heads/master@{#25746}