nv50/ir: attempt to do more constant folding on mad -> add conversion

nv50/ir: attempt to do more constant folding on mad -> add conversion

The add might actually have a 0 as an argument, which would convert it
into a mov. Make sure to detect that. Also avoid the hack of putting the
immediate directly into the instruction, instead use a mov to put it
into place and let the later LoadPropagation pass place it if possible.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>