Before, we were careful to place the zip after the last of the split
instructions but did unzip on-demand.  This changes things so that the
unzips go before all of the split instructions and the unzip comes
explicitly after all the split instructions.  As a side-effect of this
change, we now emit the split instruction from highest SIMD group to
lowest instead of low to high.  We could have kept the old behavior, but
it shouldn't matter and this made the code easier.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Cc: mesa-stable@lists.freedesktop.org