decode2: Reduce metavalue warnings

Explicitly check for undefined values and propagate them.

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

decode2.vhdl

@@ -434,6 +434,9 @@ begin
                         v.input_ov := '1';      -- need SO state if setting OV to 0
                     end if;
                 when OP_MFSPR =>
+                    if is_X(d_in.insn) then
+                        v.input_ov := 'X';
+                    else
                         case decode_spr_num(d_in.insn) is
                             when SPR_XER =>
                                 v.input_ov := '1';
@@ -441,7 +444,13 @@ begin
                                 unit := LDST;
                             when others =>
                         end case;
+                    end if;
                 when OP_MTSPR =>
+                    if is_X(d_in.insn) then
+                        v.e.output_xer := 'X';
+                        v.output_ov := 'X';
+                        v.sgl_pipe := 'X';
+                    else
                         case decode_spr_num(d_in.insn) is
                             when SPR_XER =>
                                 v.e.output_xer := '1';
@@ -456,6 +465,7 @@ begin
                         if d_in.spr_info.valid = '1' and d_in.valid = '1' then
                             v.sgl_pipe := '1';
                         end if;
+                    end if;
                 when OP_CMP | OP_MCRXRX =>
                     v.input_ov := '1';
                 when others =>