[llvm] [InstSimplify] Remove most of undef handling logic (PR #119884)

[Nuno Lopes via llvm-commits]

nunoplopes wrote:

This looks great, thank you!
As we move away from undef, most of the remaining uses are related with bitfields, so arithmetic on those isn't important (only bitwise operations).

That said, I wouldn't remove every single undef transformation because `freeze poison` with a single use can be optimized like undef in most cases. Right now, instcombine eagerly transformes those into 0, but that's suboptimal. We need to teach instcombine how to optimize `freeze poison` to eventually remove that `freeze poison -> 0` transformation.

https://github.com/llvm/llvm-project/pull/119884