[AssumptionCache] caches @llvm.experimental.guard's

As discussed in https://github.com/llvm/llvm-project/issues/59901

This change is not NFC. There is one SCEV and EarlyCSE test that have an
improved analysis/optimization case. Rest of the tests are not failing.

I've mostly only added cleanup to SCEV since that is where this issue
started. As a follow up, I believe there is more cleanup opportunity in
SCEV and other affected passes.

There could be cases where there are missed registerAssumption of
guards, but this case is not so bad because there will be no
miscompilation. AssumptionCacheTracker should take care of deleted
guards.

Differential Revision: https://reviews.llvm.org/D142330