Fix fence by disallowing x-initialization instead of all-0 mask

Setting mem_req_mask to all-zero triggers an assertion error in
mem_scheduler.  Instead, disallow initialize-by-x in instruction decode
which is the source of x-propagation.  Since this seems to only happen
in VCS, define-gate it accordingly.

This reverts commit a15f4fd483.

hw/rtl/core/VX_decode.sv

@@ -148,13 +148,21 @@ module VX_decode  #(
     always @(*) begin
 
         ex_type   = '0;
+`ifdef VCS
+        op_type   = '0;
+`else
         op_type   = 'x;
+`endif
         op_mod    = '0;
         rd_r      = '0;
         rs1_r     = '0;
         rs2_r     = '0;
         rs3_r     = '0;
+`ifdef VCS
+        imm       = '0;
+`else
         imm       = 'x;
+`endif
         use_imm   = 0;
         use_PC    = 0;
         use_rd    = 0;