[X86] Lower abdu(lhs, rhs) -> or(usubsat(lhs,rhs), usubsat(rhs,lhs))

Adds pre-SSE4 v8i16 abdu handling - we already have something similar for umax(x,y) -> add(x,usubsat(y,x)) / umin(x,y) -> sub(x,usubsat(x,y))

(I'm starting to look at adding generic TargetLowering expandABD() handling and came across this missed opportunity).

Inspiration: http://0x80.pl/notesen/2018-03-11-sse-abs-unsigned.html

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