Merged r15577 into 3.19 branch.

MIPS: Do not omit the write-barrier if the input value is a smi.

BUG=chromium:254570

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/19779002

git-svn-id: https://v8.googlecode.com/svn/branches/3.19@15758 ce2b1a6d-e550-0410-aec6-3dcde31c8c00