lima/ppir: combine varying loads in node_to_instr

lima/ppir: combine varying loads in node_to_instr

Varying loads with a single successor have a high potential to be
combined with its successor node, like ppir does for uniforms, rather
than being in a separate instruction.
Even if ppir becomes capable of combining instructions in a separate
step, combining varying loads during node_to_instr is trivial enough
that it seems to be worth doing it in this stage, and this benefits
pretty much every program that uses varyings.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Reviewed-by: Vasily Khoruzhick <anarsoul@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4535>