[llvm] [InstCombine] fold `select (trunc nuw X to i1), X, Y` to `select (trunc nuw X to i1), 1, Y` (PR #105914)

[via llvm-commits]

c8ef wrote:

> Please add an alive2 proof to the patch description.
> 
> Note that the same transform also works for trunc nsw and -1, though that's probably less practically relevant.

Added all four transformations and relevant alive2 proof.

https://github.com/llvm/llvm-project/pull/105914