[llvm] [ConstraintElim] LSHR x, n <= x when n< bitwidth (PR #209583)

[Antonio Frighetto via llvm-commits]

https://github.com/antoniofrighetto approved this pull request.

LGTM. We may want to generalize the proof to a variable shift amount: https://alive2.llvm.org/ce/z/_ZkoJA (We may freely simplify to true when the result of lshr is poison, as it would be propagated to the return value, and it is legal to refine the latter to any value).

https://github.com/llvm/llvm-project/pull/209583