nir: Tag entrypoint for easy recognition by nir_shader_get_entrypoint()

We're going to have multiple functions, so nir_shader_get_entrypoint()
needs to do something a little smarter.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/nir/nir_serialize.c

@@ -1048,6 +1048,8 @@ write_function(write_ctx *ctx, const nir_function *fxn)
       blob_write_uint32(ctx->blob, val);
    }
 
+   blob_write_uint32(ctx->blob, fxn->is_entrypoint);
+
    /* At first glance, it looks like we should write the function_impl here.
     * However, call instructions need to be able to reference at least the
     * function and those will get processed as we write the function_impls.
@@ -1072,6 +1074,8 @@ read_function(read_ctx *ctx)
       fxn->params[i].num_components = val & 0xff;
       fxn->params[i].bit_size = (val >> 8) & 0xff;
    }
+
+   fxn->is_entrypoint = blob_read_uint32(ctx->blob);
 }
 
 void