[PowerPC] Exploit vabsd on P9

Summary: Exploit vabsd* for for absolute difference of vectors on P9,
for example:
void foo (char *restrict p, char *restrict q, char *restrict t)
{
  for (int i = 0; i < 16; i++)
     t[i] = abs (p[i] - q[i]);
}
this case should be matched to the HW instruction vabsdub.

Reviewed By: steven.zhang

Differential Revision: https://reviews.llvm.org/D80271