[LoopFlatten] Forget all block and loop dispositions after flatten

Method forgetLoop only forgets expression of phi or its users. SCEV
expressions except the above mentioned may still has loop dispositions
that point to the destroyed loop, which might cause a crash.

Fixes: https://github.com/llvm/llvm-project/issues/58865

Reviewed By: nikic, fhahn

Differential Revision: https://reviews.llvm.org/D137651