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: f003859f nir: Make gl_nir_lower_samplers use gl_nir_lower_samplers_as_deref