Handle arbitrary variable as assignment LHS

(e.g., conventional compound assignment for a parameter value).

compiler/testData/ir/irText/expressions/complexAugmentedAssignment.kt

@@ -17,4 +17,17 @@ fun test2() {
     X1.x1++
     X1.X2.x2++
     X1.X2.X3.x3++
-}
+}
+
+class B(var s: Int = 0)
+
+object Host {
+    operator fun B.plusAssign(b: B) {
+        this.s += b.s
+    }
+}
+
+fun Host.test3(v: B) {
+    v += B(1000)
+}
+