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

[Rose Silicon via llvm-commits]

RSilicon wrote:

> > @topperc Do you have any objections to this PR as it is of now?
> 
> Remove the nuw handling unless you can show a real world test that benefits from it. We don't have to solve every possible case in InstCombine.

X > -C ? -1 : X + C is the canonicalization of the literal definition of sat.add.

X + C has to be nuw for this to work. 

X >= -C + 1 is basically a weird version of this. 

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