spirv: Change spirv_to_nir() to return a nir_shader

spirv_to_nir() returned the nir_function corresponding to the
entrypoint, as a way to identify it.  There's now a bool is_entrypoint
in nir_function and also a helper function to get the entry_point from
a nir_shader.

The return type reflects better what the function name suggests.  It
also helps drivers avoid the mistake of reusing internal shader
references after running NIR_PASS on it.  When using NIR_TEST_CLONE or
NIR_TEST_SERIALIZE, those would be invalidated right in the first pass
executed.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>

src/intel/vulkan/anv_pipeline.c

@@ -184,11 +184,10 @@ anv_shader_compile_to_nir(struct anv_device *device,
    };
 
 
-   nir_function *entry_point =
+   nir_shader *nir =
       spirv_to_nir(spirv, module->size / 4,
                    spec_entries, num_spec_entries,
                    stage, entrypoint_name, &spirv_options, nir_options);
-   nir_shader *nir = entry_point->shader;
    assert(nir->info.stage == stage);
    nir_validate_shader(nir, "after spirv_to_nir");
    ralloc_steal(mem_ctx, nir);