[LoopUnroll] Forget block and loop dispositions during unrolling.

After unrolling 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.

I am planning on reviewing/updating the existing uses of
forgetLoopDispositions to check if they should be replaced by
forgetBlockAndLoopDispositions.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D134612