Revert "[SCEV] rewriteLoopExitValues(): even if have hard uses, still rewrite if cheap (PR44668)"

As discussed in post-commit review in https://reviews.llvm.org/D73501
if the goal of this is to help vectorizer, then we should actually
be teaching vectorizer to do this, because right now this rewrite
is still budget-limited, which isn't what we'd want.

Additionally, while the rest of the patch series was universally profitable,
this particular patch is reportedly (https://reviews.llvm.org/D73501#1905171)
exposing cost-modeling issues on ARM.

So let's just back this particular patch out. Once there's an undo transform,
this could be considered for reintegration.

This reverts commit 44edc6fd.