[mips] Fix inefficient code generation.

This patch eliminates the need to emit a constant move instruction when this
pattern is matched:

(select (setgt a, Constant), T, F)

The pattern above effectively turns into this:

(conditional-move (setlt a, Constant + 1), F, T)



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176384 91177308-0d34-0410-b5e6-96231b3b80d8