[NewPM][SimpleLoopUnswitch] Add option to not trivially unswitch

To help with debugging non-trivial unswitching issues.

Don't care about the legacy pass, nobody is using it.

If a pass's string params are empty (e.g. "simple-loop-unswitch"), don't
default to the empty constructor for the pass params. We should still
let the parser take care of it in case the parser has its own defaults.

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D105933