opt_expr: Propagate constants to port connections.

This adds one simple piece of functionality to opt_expr: when a cell
port is connected to a fully-constant signal (as determined by sigmap),
the port is reconnected directly to the constant value.  This is just
enough optimization to fix the "non-constant $meminit input" problem
without requiring a full opt_clean or a separate pass.

tests/opt/opt_expr_constconn.v

@@ -0,0 +1,8 @@
+module top(...);
+
+input [7:0] A;
+output [7:0] B;
+wire [7:0] C = 3;
+assign B = A + C;
+
+endmodule