nir: Allow a non-existent sampler deref in nir_lower_samplers_as_deref

nir: Allow a non-existent sampler deref in nir_lower_samplers_as_deref

GL_ARB_gl_spirv does not provide a sampler deref for e.g. texelFetch(),
so we can't assume that both are present and identical.  Simply lower
each if it is present.

Fixes regressions in GL_ARB_gl_spirv tests since I switched everyone to
using this pass.  Thanks to Alejandro Piñeiro for catching these.

Fixes: f003859f97c nir: Make gl_nir_lower_samplers use gl_nir_lower_samplers_as_deref
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Tested-by: Alejandro Piñeiro <apinheiro@igalia.com>