[DAG] Add visitABD optimizations

This adds basic a visitABD to optimize ABDS and ABDU nodes, similar to the
existing visitAVG method.

The fold I was initially interested in was folding shuffles though the binop.
This also:
- Marks ABDS and ABDU as commutative binops (https://alive2.llvm.org/ce/z/oCDogb
  and https://alive2.llvm.org/ce/z/7zrs86).
- Add reassociative folds.
- Add constant folding using max(x,y)-min(x,y)
- Canonicalizes constants to the RHS
- Folds abds x, 0 -> abs(x) (https://alive2.llvm.org/ce/z/4ZEibv)
- Folds abdu x, 0 -> x (https://alive2.llvm.org/ce/z/J_rKqx)
- Folds abd x, undef -> 0 (https://alive2.llvm.org/ce/z/NV6Nsv and
  https://alive2.llvm.org/ce/z/vs92hu).

Differential Revision: https://reviews.llvm.org/D143193