nv50/ir: process texture offset sources as regular sources

nv50/ir: process texture offset sources as regular sources

With ARB_gpu_shader5, texture offsets can be any source, including TEMPs
and IN's. Make sure to process them as regular sources so that we pick
up masks, etc.

This should fix some CTS tests that feed offsets directly to
textureGatherOffset, and we were not picking up the input use, thus not
advertising it in the shader header.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Tested-by: Dave Airlie <airlied@redhat.com>
Cc: 12.0 13.0 <mesa-stable@lists.freedesktop.org>