intel/fs: Split instructions low to high in lower_simd_width

Commit 0d905597 fixed an issue with the placement of the zip and unzip
instructions.  However, as a side-effect, it reversed the order in which
we were emitting the split instructions so that they went from high
group to low instead of low to high.  This is fine for most things like
texture instructions and the like but certain render target writes
really want to be emitted low to high.  This commit just switches the
order back around to be low to high.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Fixes: 0d905597 "intel/fs: Be more explicit about our placement of [un]zip"
(cherry picked from commit d5b617a2)