[PowerPC] fix incorrect vectorization of abs() on POWER9

[PowerPC] fix incorrect vectorization of abs() on POWER9

Vectorized loops with abs() returns incorrect results on POWER9. This patch fixes it.
For example the following code returns negative result if input values are negative though it sums up the absolute value of the inputs.

int vpx_satd_c(const int16_t *coeff, int length) {
  int satd = 0;
  for (int i = 0; i < length; ++i) satd += abs(coeff[i]);
  return satd;
}

This problem causes test failures for libvpx.
For vector absolute and vector absolute difference on POWER9, LLVM generates VABSDUW (Vector Absolute Difference Unsigned Word) instruction or variants.
Since these instructions are for unsigned integers, we need adjustment for signed integers.
For abs(sub(a, b)), we generate VABSDUW(a+0x80000000, b+0x80000000). Otherwise, abs(sub(-1, 0)) returns 0xFFFFFFFF(=-1) instead of 1. For abs(a), we generate VABSDUW(a+0x80000000, 0x80000000).

Differential Revision: https://reviews.llvm.org/D45522

llvm-svn: 330497