[DAG] Legalize abs(x) -> smax(x,sub(0,x)) iff smax/sub are legal

If smax() is legal, this is likely to result in smaller codegen expansion for abs(x) than the xor(add,ashr) method.

This is also what PowerPC has been doing for its abs implementation, so it lets us get rid of a load of custom lowering code there (and which was never updated when they added smax lowering).

Alive2: https://alive2.llvm.org/ce/z/xRk3cD

Differential Revision: https://reviews.llvm.org/D92095