[llvm] [InstCombine] Fold `select Cond, not X, X` into `Cond ^ X` (PR #93591)

[via llvm-commits]

goldsteinn wrote:

> > Im somewhat partial to #90089. In general would prefer to use existing analysis logic (`isImpliedCondition...`) than this bespoke function. Is there anything this function can do that other cant?
> 
> No. I just use this simplified helper function to reduce compile-time overhead. See [#90089 (comment)](https://github.com/llvm/llvm-project/pull/90089#discussion_r1616325307)

Okay, compile time is impact seems like a reasonable motivation.

https://github.com/llvm/llvm-project/pull/93591