Fix for PR3040:

Fix for PR3040:

The CC was changed, but wasn't checked to see if it was legal if the DAG
combiner was being run after legalization. Threw in a couple of checks just to
make sure that it's okay. As far as the PR is concerned, no back-end target
actually exhibited this problem, so there isn't an associated testcase.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@59035 91177308-0d34-0410-b5e6-96231b3b80d8