[X86][SSE] Use (V)PHMINPOSUW for vXi8 SMAX/SMIN/UMAX/UMIN horizontal reductions (PR32841)

Extension to D39729 which performed this for vXi16, with the same bit flipping to handle SMAX/SMIN/UMAX cases, vXi8 UMIN horizontal reductions can be performed.

This makes use of the fact that by performing a pair-wise i8 SHUFFLE/UMIN before PHMINPOSUW, we both get the UMIN of each pair but also zero-extend the upper bits ready for v8i16.

Differential Revision: https://reviews.llvm.org/D41294

llvm-svn: 321070