spirv/nir: add support for ImageGatherExtended
spirv/nir: add support for ImageGatherExtended

The strategy is to do the same thing that the GLSL lower_offset_arrays
pass does - create 4 separate texture gather ops, one per offset, and
read in the results from each gather's w component to recreate the
desired result.

Signed-off-by: Ilia Mirkin <bcbbb7aa705ec3a5fc5824c01a845186c3d62fca@alum.mit.edu>
Reviewed-by: Dave Airlie <f2295d84e358395675bc8031be58672073ae065e@redhat.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler