nv50/ir: handle insn not being there for definition of CVT arg

This can happen if it's e.g. a uniform or a function argument.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111217


Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit 3e468ff2)