The patch optimizes the generation of code for kMips[64]Tst in code-generator-mips[64].cc.
We allow usage of immediate for kMips[64]Tst in VisitWordCompare in order to remove
an unnecessary immediate load to register. This fix is motivated by Richards benchark
in Octane benchmark suite, since it is used a lot there.

BUG=

Review-Url: https://codereview.chromium.org/2503833002
Cr-Commit-Position: refs/heads/master@{#40992}