[PowerPC] Improve vec_abs on P9

Improve the current vec_abs support on P9, generate ISD::ABS node for vector types,
combine ABS node to VABSD node for some special cases to make use of P9 VABSD* insns,
do custom lowering to vsub(vneg later)+vmax if it has no combination opportunity.

Differential Revision: https://reviews.llvm.org/D54783

llvm-svn: 349437