[PowerPC]Exploit P9 vabsdu for unsigned vselect patterns

For type v4i32/v8ii16/v16i8, do following transforms:
  (vselect (setcc a, b, setugt), (sub a, b), (sub b, a)) -> (vabsd a, b)
  (vselect (setcc a, b, setuge), (sub a, b), (sub b, a)) -> (vabsd a, b)
  (vselect (setcc a, b, setult), (sub b, a), (sub a, b)) -> (vabsd a, b)
  (vselect (setcc a, b, setule), (sub b, a), (sub a, b)) -> (vabsd a, b)

Differential Revision: https://reviews.llvm.org/D55812

llvm-svn: 349599