[SCEV] Prove condition invariance via context

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