i965/vec4: Add and use byte-MOV instruction for unpack 4x8.

i965/vec4: Add and use byte-MOV instruction for unpack 4x8.

Previously we were using a B/UB source in an Align16 instruction, which
is illegal. It for some reason works on all platforms, except Broadwell.

Cc: "10.5" <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=86811
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>