[LoopFlatten] Mark loop analyses as preserved

LoopFlatten does preserve loop analyses (DT, LI and SCEV), but
currently doesn't mark them as preserved in the NewPM (they are
marked as preserved in the LegacyPM). I think this doesn't really
have an effect in the end because the loop pass adaptor will just
assume they're preserved anyway, but let's be explicit about this
for the sake of clarity.

Differential Revision: https://reviews.llvm.org/D111328