SelectionDAG: slightly refactor DAGCombiner::visitSELECT_CC to avoid redudant checks...

This doesn't really effect performance due to all the relevant calls being transparent but is clearer. 


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184027 91177308-0d34-0410-b5e6-96231b3b80d8