This leaves lftr.ll alone, because there is a suspicious test diff.

Initial implementation had too weak requirements to positive/negative
range crossings. Not crossing zero with nuw is not enough for two reasons:

- If ArLHS has negative step, it may turn from positive to negative
  without crossing 0 boundary from left to right (and crossing right to
  left doesn't count for unsigned);
- If ArLHS crosses SINT_MAX boundary, it still turns from positive to
  negative;

In fact we require that ArLHS always stays non-negative or negative,
which an be enforced by the following set of preconditions:

- both nuw and nsw;
- positive step (looks liftable);

Because of positive step, boundary crossing is only possible from left
part to the right part. And because of no-wrap flags, it is guaranteed
to never happen.

This reverts commit a3d1fb3b59b473e4f262a05f187de6474b7721e7.

Reverting until investigation of https://github.com/llvm/llvm-project/issues/57247
has concluded.

Contextual knowledge may be used to prove invariance of some conditions.
For example, in this case:
```
  ; %len >= 0
  guard(%iv = {start,+,1}<nuw> <s %len)
  guard(%iv = {start,+,1}<nuw> <u %len)
```
the 2nd check always fails if `start` is negative and always passes otherwise.

It looks like there are more opportunities of this kind that are still to be
implemented in the future.

Differential Revision: https://reviews.llvm.org/D129753
Reviewed By: apilipenko

This patch teaches SCEV two implication rules:

  x <u y && y >=s 0 --> x <s y,
  x <s y && y <s 0 --> x <u y.

And all equivalents with signs/parts swapped.

Differential Revision: https://reviews.llvm.org/D110517
Reviewed By: nikic

More similar cases to see that the opt we are trying to make is generic enough.

Previously I've added tests that require context for inference, but it
seems tha SCEV can't prove same facts even when the context isn't required.