[mips] Add some comments related to the optimization performed in performSELECTCombine.
The structure of the code was slightly modified so that the next patch is easier to read/review.

No functional changes.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196496 91177308-0d34-0410-b5e6-96231b3b80d8