[DAGCombiner] `visitFREEZE()`: fix handling of no maybe-poison ops

The original code was confusing. It was stripping poison-generating flags,
but the comments were saying that doing so was a TODO.

If the poison-generating flags are present, then even if all operands
are guaranteed not to be undef or poison, the whole operation may still
produce undef or poison. We can still deal with that case,
and we already do deal with it in fact, by also dropping those flags.

Refs. https://github.com/llvm/llvm-project/issues/59676