i965/vec4: Import helpers to convert vectors into arrays and back.

i965/vec4: Import helpers to convert vectors into arrays and back.

These functions handle the conversion of a vec4 into the form expected
by the dataport unit in message and message return payloads.  The
conversion is not always trivial because some messages don't support
SIMD4x2 for some generations, in which case a strided copy may be
necessary.

v2: Split from the FS implementation.
v3: Rewrite to avoid evil array_reg, emit_collect and emit_zip.

Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>