For connection `assign a = b;`, `sigmap(a)` returns `b`. This is
exactly the opposite of the desired canonicalization for driven bits.
Consider the following code:
module foo(inout a, b);
assign a = b;
endmodule
module bar(output c);
foo f(c, 1'b0);
endmodule
Before this commit, the inout ports would be swapped after flattening
(and cause a crash while attempting to drive a constant value).
This issue was introduced in 9f772eb9.
Fixes#2183.
