nv50/ir: propagate modifier to right arg when const-folding mad
An immediate has to be the second arg of an ADD operation. However we were mistakenly propagating the modifier of the non-folded value to the folded immediate argument. Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91117 Signed-off-by:Ilia Mirkin <imirkin@alum.mit.edu> Cc: "10.5 10.6" <mesa-stable@lists.freedesktop.org> (cherry picked from commit ad62ec83)