Reimplement opt_share to work on $alu and $pmux

tests/opt/opt_share_cat_multiuser.v

@@ -0,0 +1,22 @@
+module opt_share_test(
+               input [15:0] 		 a,
+               input [15:0] 		 b,
+               input [15:0] 		 c,
+               input [15:0] 		 d,
+               input 						 sel,
+               output reg [47:0] res,
+               );
+
+  wire [15:0] 									 add_res = a+b;
+  wire [15:0] 									 sub_res = a-b;
+  wire [31: 0] 									 cat1 = {add_res, c+d};
+  wire [31: 0] 									 cat2 = {sub_res, c-d};
+
+  always @* begin
+    case(sel)
+      0: res = {cat1, add_res};
+      1: res = {cat2, add_res};
+    endcase
+  end
+
+endmodule