v3d: Make sure that QPU instruction-has-a-dest matches VIR.

Found when debugging register spilling -- we would try to spill the dest
of a STVPMV, inserting spill code after entering the last segment.  In
fact, we were likely to to choose to do this, given that the STVPMV "dest"
temp was never read from, making it cheap to spill.

Cc: "18.2" <mesa-stable@lists.freedesktop.org>
(cherry picked from commit f2c0d310)