compiler/nir: Add support for variable initialization from a pointer

Add a pointer_initializer field to nir_variable analogous to
constant_initializer, which can be used to initialize the nir_variable
to a pointer to another nir_variable. Just like the
constant_initializer, the pointer_initializer gets eliminated in the
nir_lower_constant_initializers pass.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3047>

src/compiler/nir/nir_print.c

@@ -558,6 +558,8 @@ print_var_decl(nir_variable *var, print_state *state)
       print_constant(var->constant_initializer, var->type, state);
       fprintf(fp, " }");
    }
+   if (var->pointer_initializer)
+      fprintf(fp, " = &%s", get_var_name(var->pointer_initializer, state));
 
    fprintf(fp, "\n");
    print_annotation(state, var);