nvc0/ir: fix collection of first uses for texture barrier insertion

nvc0/ir: fix collection of first uses for texture barrier insertion

One of the places we have to insert texbars is in situations where the
result of the tex gets overwritten by a different instruction (e.g. in a
conditional statement). However in some situations it can actually
appear as though the original tex itself is an overwriting instruction.
This can naturally never really happen, so just ignore the tex
instruction when it comes up.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90347
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "10.5 10.6" <mesa-stable@lists.freedesktop.org>