Generate equals()/hashCode(): Fix hashCode() implementation

#KT-11638 Fixed

idea/testData/codeInsight/generate/equalsWithHashCode/singleVarWithJavaSuperClass.kt.after

@@ -17,7 +17,7 @@ class A(val n: Int) : X() {
 
     override fun hashCode(): Int{
         var result = super.hashCode()
-        result += 31 * result + n
+        result = 31 * result + n
         return result
     }
 }