spirv: Use nir_deref_type_null for OpConstantNull

Use for SSBOs and shared memory, which will exercise the two main
codepaths we have for I/O lowering.
14 jobs for !769 with r/nir-deref-type-null in 7 minutes and 9 seconds (queued for 5 seconds)
detached
Status Job ID Name Coverage
  Containers Build
passed #271829
debian

00:00:17

 
  Build+Test
passed #271831
meson-clang

00:06:49

passed #271837
meson-gallium-clover-llvm

00:02:32

passed #271838
meson-gallium-clover-llvm39

00:03:48

passed #271836
meson-gallium-drivers-other

00:04:45

passed #271835
meson-gallium-radeonsi

00:02:44

passed #271839
meson-gallium-st-other

00:02:44

passed #271830
meson-gallium-swr

00:04:06

passed #271834
meson-glvnd

00:03:34

passed #271833
meson-loader-classic-dri

00:02:57

passed #271832
meson-vulkan

00:02:25

passed #271841
scons-llvm

00:02:55

passed #271840
scons-nollvm

00:04:55

passed #271842
scons-swr

00:06:50