[SCEV] Fully invalidate SCEVUnknown on RAUW

When a SCEVUnknown gets RAUWd, we currently drop it from the folding
set, but don't forget memoized values. I believe we should be
treating RAUW the same way as deletion here and invalidate all
caches and dependent expressions.

I don't have any specific cases where this causes issues right now,
but it does address the FIXME in https://reviews.llvm.org/D119488.

Differential Revision: https://reviews.llvm.org/D120033