register_file: Report value being written before asserting it's not X

If a bug causes an indeterminate value to be written to a GPR, an
assert causes simulation to abort.  Move the assert after the report
of the GPR index and value so that we get to know what the bad value
is before the simulation terminates.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

register_file.vhdl

@@ -40,12 +40,12 @@ begin
     begin
         if rising_edge(clk) then
             if w_in.write_enable = '1' then
-                assert not(is_x(w_in.write_data)) and not(is_x(w_in.write_reg)) severity failure;
 		if w_in.write_reg(5) = '0' then
 		    report "Writing GPR " & to_hstring(w_in.write_reg) & " " & to_hstring(w_in.write_data);
 		else
 		    report "Writing GSPR " & to_hstring(w_in.write_reg) & " " & to_hstring(w_in.write_data);
 		end if;
+                assert not(is_x(w_in.write_data)) and not(is_x(w_in.write_reg)) severity failure;
                 registers(to_integer(unsigned(w_in.write_reg))) <= w_in.write_data;
             end if;
         end if;