[FIR] Extract LHS receiver of assignment operator statements

#KT-53490 Fixed

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

@@ -0,0 +1,52 @@
+// TARGET_BACKEND: JVM_IR
+// IGNORE_BACKEND_K1: JVM_IR
+
+var cnt = 0
+
+fun A?.getA(): A? {
+    cnt++
+    return this
+}
+
+fun get0(): Int {
+    cnt++
+    return 0
+}
+
+class A
+
+var A?.b: A?
+    get() {
+        return this
+    }
+    set(v) {
+    }
+
+operator fun A?.get(i: Int): A? = this
+operator fun A?.set(i: Int, v: A?): A? {
+    return this
+}
+
+operator fun A?.plus(a: A?) = this
+
+fun test(a: A?) {
+    a?.getA().b += null
+    a.getA().b += null
+
+    a?.b[get0()] += null
+    a.b[get0()] += null
+
+    a?.b[get0()][get0()] += null
+    a.b[get0()][get0()] += null
+}
+
+fun box(): String {
+    test(null)
+    if (cnt != 4) return "fail 1: $cnt"
+
+    cnt = 0
+    test(A())
+    if (cnt != 8) return "fail 2: $cnt"
+
+    return "OK"
+}