[llvm] [InstCombine] simplify average of lsb (PR #95684)

[Hendrik Hübner via llvm-commits]

HendrikHuebner wrote:

@c8ef I was already assigned to this and still planning to work on it. Please ask first before you take issues that are already assigned to someone else.

https://github.com/llvm/llvm-project/pull/95684