Allow to call += on a val

compiler/testData/codegen/box/operatorConventions/assignmentOperations.kt

@@ -0,0 +1,36 @@
+class A() {
+  var x = 0
+}
+
+fun A.plusAssign(y: Int) { x += y }
+fun A.minusAssign(y: Int) { x -= y }
+fun A.timesAssign(y: Int) { x *= y }
+fun A.divAssign(y: Int) { x /= y }
+fun A.modAssign(y: Int) { x %= y }
+
+fun box(): String {
+  val original = A()
+  val a = original
+
+  a += 1
+  if (!(a identityEquals original)) return "Fail 1: $a !== $original"
+  if (a.x != 1) return "Fail 2: ${a.x} != 1"
+
+  a -= 2
+  if (!(a identityEquals original)) return "Fail 3: $a !== $original"
+  if (a.x != -1) return "Fail 4: ${a.x} != -1"
+
+  a *= -10
+  if (!(a identityEquals original)) return "Fail 5: $a !== $original"
+  if (a.x != 10) return "Fail 6: ${a.x} != 10"
+
+  a /= 3
+  if (!(a identityEquals original)) return "Fail 7: $a !== $original"
+  if (a.x != 3) return "Fail 8: ${a.x} != 3"
+
+  a %= 2
+  if (!(a identityEquals original)) return "Fail 9: $a !== $original"
+  if (a.x != 1) return "Fail 10: ${a.x} != 1"
+
+  return "OK"
+}