The legacy LoopUnswitch pass is only used in the legacy pass manager
pipeline, which is deprecated.
The NewPM replacement is SimpleLoopUnswitch and I think it is time to
remove the legacy LoopUnswitch code.
Fixes#31000.
Reviewed By: aeubanks, Meinersbur, asbirlea
Differential Revision: https://reviews.llvm.org/D124376