opt_expr: Fix handling of $_XNOR_ cells with A = B.

Fixes #2311.

passes/opt/opt_expr.cc

@@ -604,7 +604,7 @@ void replace_const_cells(RTLIL::Design *design, RTLIL::Module *module, bool cons
 				if (cell->type.in(ID($xnor), ID($_XNOR_))) {
 					cover("opt.opt_expr.const_xnor");
 					// For consistency since simplemap does $xnor -> $_XOR_ + $_NOT_
-					int width = cell->getParam(ID::Y_WIDTH).as_int();
+					int width = GetSize(cell->getPort(ID::Y));
 					replace_cell(assign_map, module, cell, "const_xnor", ID::Y, SigSpec(RTLIL::State::S1, width));
 					goto next_cell;
 				}

tests/opt/bug2311.ys

@@ -0,0 +1,14 @@
+read_verilog -icells << EOT
+
+module top(...);
+
+input A;
+output Y;
+
+$_XNOR_ x (.A(A), .B(A), .Y(Y));
+
+endmodule
+
+EOT
+
+equiv_opt -assert opt_expr