[llvm] [InstCombine] Treat umax as select(icmp eq x, 0), 1, x) in binop select fold. (PR #65978)

[Nikita Popov via llvm-commits]

nikic wrote:

I'm not a fan of the special undef handling here. We should either insert the necessary freeze, or, if that covers your motivating case, we can start with guarding the transform under isGuaranteedNotToBeUndefOrPoison().

https://github.com/llvm/llvm-project/pull/65978