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

#KT-11638 Fixed

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

@@ -26,9 +26,9 @@ class A(val n: Int, val s: String) : X() {
 
     override fun hashCode(): Int{
         var result = super.hashCode()
-        result += 31 * result + n
-        result += 31 * result + s.hashCode()
-        result += 31 * result + f.hashCode()
+        result = 31 * result + n
+        result = 31 * result + s.hashCode()
+        result = 31 * result + f.hashCode()
         return result
     }
 }