[X86][SSE] Use (V)PHMINPOSUW for vXi16 SMAX/SMIN/UMAX/UMIN horizontal reductions (PR32841)

(V)PHMINPOSUW determines the UMIN element in an v8i16 input, with suitable bit flipping it can also be used for SMAX/SMIN/UMAX cases as well.

This patch matches vXi16 SMAX/SMIN/UMAX/UMIN horizontal reductions and reduces the input down to a v8i16 vector before calling (V)PHMINPOSUW.

A later patch will use this for v16i8 reductions as well (PR32841).

Differential Revision: https://reviews.llvm.org/D39729

llvm-svn: 318917