nir/spirv: Fix assert when unsampled OpTypeImage has unknown 'Depth'

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

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>