[SCEV] Verify contents of loop disposition cache

It seems that it is sometimes broken. Initial motivation for this was
investigation of https://github.com/llvm/llvm-project/issues/56260, but
it also seems that we have found an unrelated bug in LoopFusion that leaves
broken caches.

Differential Revision: https://reviews.llvm.org/D134158
Reviewed By: nikic