[LoopDeletion] Forget block and loop dispositions after deleting loop.

After deleting a loop, the block and loop dispositions need to be
cleared. As we don't know which SCEVs in the loop/blocks may be
impacted, completely clear the cache. This should also fix some cases
where deleted loops remained in the LoopDispositions cache.

This fixes a verification failure surfaced by D134531.

Reviewed By: mkazantsev

Differential Revision: https://reviews.llvm.org/D134613