nvc0/ir: return 0 in imageLoad on incomplete textures

We already guarded all OP_SULDP against out of bound accesses, but we
ended up just reusing whatever value was stored in the dest registers.

Fixes CTS test shader_image_load_store.incomplete_textures

v2: fix for loads not ending up with predicates (bindless_texture)
v3: fix replacing the def

Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
(cherry picked from commit c3325097)
Conflicts resolved by Dylan

Conflicts:
	src/gallium/drivers/nouveau/codegen/nv50_ir_lowering_nvc0.h