It's common to delete some instructions after using SCEVExpander,
while it is still live (but will not be used afterwards). In that
case, the AssertingVH may trigger. Replace it with a PoisoningVH
so that we only detect the case where the SCEVExpander actually is
used in a problematic fashion after the instruction removal.
The alternative would be to add clear() calls to more code paths.
Fixes https://github.com/llvm/llvm-project/issues/83404.
6409c21857