nir/spirv: Fix assert when unsampled OpTypeImage has unknown 'Depth'
'dxc' hlsl-to-spirv compiler appears to emit 2 (Unknown) in the depth field, when the image is not sampled and the value is not needed.
Previously, shaders failed with:
SPIR-V parsing FAILED: In file ../src/compiler/spirv/spirv_to_nir.c:1412 !is_shadow 632 bytes into the SPIR-V binary