Revert "[SCEV] Model `ashr exact x, C` as `(abs(x) EXACT/u (1<<C)) * signum(x)`"

As being discussed in https://reviews.llvm.org/D100721,
this modelling is lossy, we can't reconstruct `ash`/`ashr exact`
from it, which means that whenever we actually expand the IR,
we've just pessimized the code..

It would be good to model this pattern, after all it comes up every time
you want to compute a distance between two pointers, but not at this cost.

This reverts commit ec54867d.