[X86] Codegen for one of 'absolute difference' patterns is suboptimal

[LebedevRI]

As being pointed out in halide/Halide#6545
we don't codegen a < b ? b - a : a - b as well as we do max(a, b) - min(a, b)
https://godbolt.org/z/MvMj9MPW9
https://alive2.llvm.org/ce/z/pVHePi

The question is, what do we think is more canonical in IR?

[llvmbot]

@llvm/issue-subscribers-backend-X86

[rotateright]

Should we consider the form with min/max intrinsics? (we'll canonicalize to them some day...)
https://alive2.llvm.org/ce/z/kCpnF8

We'll need backend optimizations no matter which form we choose in IR. Even within the scalar or vector subsets, x86 may be suboptimal depending on type (for example, no cmov for i8 directly).

[RKSimon]

CC @RKSimon

[RKSimon]

https://reviews.llvm.org/D144789 will add the select(icmp_gt(x,y),sub(x,y),sub(y,x)) -> abd(x,y) fold - although we don't support scalars