[x86] use a single shufps for 256-bit vectors when it can save instructions

This is the 256-bit counterpart to the 128-bit transform checked in here:
https://reviews.llvm.org/rL289837

This patch is based on the draft by @sroland (Roland Scheidegger) that is
attached to PR27885:
https://llvm.org/bugs/show_bug.cgi?id=27885