[mips] Optimize conditional moves where RHS is zero.

[mips] Optimize conditional moves where RHS is zero.

Summary:
When the RHS of a conditional move node is zero, we can utilize the $zero
register by inverting the conditional move instruction and by swapping the
order of its True/False operands.

Reviewers: dsanders

Differential Revision: http://reviews.llvm.org/D7945

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230956 91177308-0d34-0410-b5e6-96231b3b80d8