Teach LoopUnswitch not to perform non-trivial unswitching on loops containing converg...

Teach LoopUnswitch not to perform non-trivial unswitching on loops containing convergent operations.

Doing so could cause the post-unswitching convergent ops to be
control-dependent on the unswitch condition where they were not before.
This check could be refined to allow unswitching where the convergent
operation was already control-dependent on the unswitch condition.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@249874 91177308-0d34-0410-b5e6-96231b3b80d8