hashlib: allow forcing Hasher state, use it for IdString trivial hashing

kernel/hashlib.h

@@ -132,6 +132,13 @@ class Hasher {
 		state ^= t;
 	}
 
+	void force(hash_t new_state) {
+		state = new_state;
+	}
+
+	bool is_new() const {
+		return state == Hasher().state;
+	}
 };
 
 template<typename T>

kernel/rtlil.h

@@ -361,6 +361,12 @@ namespace RTLIL
 		}
 
 		Hasher hash_acc(Hasher h) const {
+			// If we're starting a hashing sequence, simply start with unhashed ID
+			if (h.is_new()) {
+				h.force((Hasher::hash_t) index_);
+				return h;
+			}
+
 			return hash_ops<int>::hash_acc(index_, h);
 		}