[Power9] Exploit vector absolute difference instructions on Power 9

Power 9 has instructions to do absolute difference (VABSDUB, VABSDUH, VABSDUW)
for byte, halfword and word. We should take advantage of these.

Differential Revision: https://reviews.llvm.org/D34684

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@309876 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed