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

#KT-11638 Fixed

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

@@ -22,8 +22,8 @@ class A(val n: IntArray?, val s: Array<String>?) {
 
     override fun hashCode(): Int{
         var result = n?.let { Arrays.hashCode(it) } ?: 0
-        result += 31 * result + (s?.let { Arrays.hashCode(it) } ?: 0)
-        result += 31 * result + f.hashCode()
+        result = 31 * result + (s?.let { Arrays.hashCode(it) } ?: 0)
+        result = 31 * result + f.hashCode()
         return result
     }
 }