[llvm] [InstCombine] Canonicalize more saturated add variants (PR #100008)

[Rose Silicon via llvm-commits]

RSilicon wrote:

> This PR was pretty close to an acceptable state the last time I looked at it. Now you have once again added extra changes that nobody asked for, and moved it away from acceptance again.

The change to AllowPoison is warranted. See the tests and Alive2.

https://github.com/llvm/llvm-project/pull/100008