nv50/ir: avoid creating instructions that can't be emitted

nv50/ir: avoid creating instructions that can't be emitted

When constant folding a MAD operation, we first fold the multiply and
generate an ADD. However we do so without making sure that the immediate
can be handled in the saturate case. If it can't, load the immediate in
a separate instruction.

Reported-by: Tiziano Bacocco <tizbac2@gmail.com>
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "10.2 10.3" <mesa-stable@lists.freedesktop.org>