[X86][SSE] Use PSADBW to improve vXi8 sum reduction (PR42674)

As detailed on PR42674, we can reduce a vXi8 down until we have the final <8 x i8>, and then use PSADBW with zero, to sum those values. We then extract the bottom i8, discarding any overflow from the upper bits of the i16 result.

llvm-svn: 366636