[SCEV] Use lshr in implications

This patch adds support for implication inference logic for the
following pattern:
```
  lhs < (y >> z) <= y, y <= rhs --> lhs < rhs
```
We should be able to use the fact that value shifted to right is
not greater than the original value (provided it is non-negative).

Differential Revision: https://reviews.llvm.org/D116150
Reviewed-By: apilipenko