spirv: Fix a couple of image atomic load/store bugs

For one thing, the NIR opcodes for image load/store always take and
return a vec4 value regardless of the image type.  We need to fix up
both the source and destination to handle it.  For another thing, we
weren't actually setting up a destination in the OpAtomicLoad case.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit 5e030dea)
Conflicts resolved by Dylan

Conflicts:
	src/compiler/spirv/spirv_to_nir.c