Refactor common parts of SAT-using optimizations into a helper.

This also aligns the functionality:

- in all cases, the onehot attribute is used to create appropriate
  constraints (previously, opt_dff didn't do it at all, and share
  created one-hot constraints based on $pmux presence alone, which
  is unsound)
- in all cases, shift and mul/div/pow cells are now skipped when
  importing the SAT problem (previously only memory_share did this)
  — this avoids creating clauses for hard cells that are unlikely
  to help with proving the UNSATness needed for optimization

kernel/modtools.h

@@ -380,9 +380,11 @@ struct ModWalker
 		}
 	}
 
-	ModWalker(RTLIL::Design *design) : design(design), module(NULL)
+	ModWalker(RTLIL::Design *design, RTLIL::Module *module = nullptr) : design(design), module(NULL)
 	{
-            ct.setup(design);
+		ct.setup(design);
+		if (module)
+			setup(module);
 	}
 
 	void setup(RTLIL::Module *module, CellTypes *filter_ct = NULL)