nir/spirv: tg4 requires a sampler

Gather operations in both GLSL and SPIR-V require a sampler. Fixes
gathers returning garbage when using separate texture/samplers (on AMD,
was using an invalid sampler descriptor).

Signed-off-by: Alex Smith <asmith@feralinteractive.com>
Cc: "17.2 17.3" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit 4122d008)