Reimplement opt_share to work on $alu and $pmux

tests/opt/opt_share_extend.v

@@ -0,0 +1,19 @@
+module opt_share_test(
+											input signed [7:0] 			 a,
+											input signed [10:0] 		 b,
+											input signed [15:0] 		 c,
+											input [1:0] 						 sel,
+											output reg signed [15:0] res
+											);
+
+
+  always @* begin
+    case(sel)
+      0: res = a + b;
+      1: res = a - b;
+      2: res = a + c;
+      default: res = 16'bx;
+    endcase
+  end
+
+endmodule