Teach DAG combine to handle vector logical operations with vectors of all 1s or all...

Teach DAG combine to handle vector logical operations with vectors of all 1s or all 0s. These cases can show up when vectors are split for legalizing. Fix some tests that were dependent on these cases not being combined.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@169684 91177308-0d34-0410-b5e6-96231b3b80d8