[LoopDeletion] Forget loop before setting values to undef

After D71539, we need to forget the loop before setting the incoming
values of phi nodes in exit blocks, because we are looking through those
phi nodes now and the SCEV expression could depend on the loop phi. If
we update the phi nodes before forgetting the loop, we miss those users
during invalidation.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D88167