Make the call to compare_signals() easier to read.

The negation here is confusing. The intent of the code is "if `s1` is preferred
over `s2` as the canonical `SigBit` for this signal, make `s1` the canonical `SigBit`
in `assign_map`", so write the code that way instead of "if `s2` is not preferred
over `s1` ...".

This doesn't change any behavior now that `compare_signals()` is a total order,
i.e. `s1` is preferred over `s2`, `s2` is preferred over `s1`, or `s1` and `s2` are equal.
Now, when `s1` and `s2` are equal, we don't call `assign_map.add(s1)`, but that's
already a noop in that case.

passes/opt/opt_clean.cc

@@ -346,7 +346,7 @@ bool rmunused_module_signals(RTLIL::Module *module, bool purge_mode, bool verbos
 		RTLIL::Wire *wire = it.second;
 		for (int i = 0; i < wire->width; i++) {
 			RTLIL::SigBit s1 = RTLIL::SigBit(wire, i), s2 = assign_map(s1);
-			if (!compare_signals(s1, s2, register_signals, connected_signals, direct_wires))
+			if (compare_signals(s2, s1, register_signals, connected_signals, direct_wires))
 				assign_map.add(s1);
 		}
 	}