Revert "Revert r212640, "Add trunc (select c, a, b) -> select c (trunc a), (trunc b) combine.""

Don't try to convert the select condition type.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212750 91177308-0d34-0410-b5e6-96231b3b80d8