nv50/ir: propagate modifier to right arg when const-folding mad

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 ad62ec8316a926682958e7ab52639992867c3755)