SLPVectorizer: When sorting by domination for CSE don't assert on unreachable code.

There is no total ordering if the CFG is disconnected. We don't care if we
catch all CSE opportunities in dead code either so just exclude ignore them in
the assert.

PR19646

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208461 91177308-0d34-0410-b5e6-96231b3b80d8