Rework support of Iterator.iterator optimization in ForLoopsLowering

- Fix the predicate used for finding the member function `next`: check
  that there's no extension receiver parameter, don't filter out
  abstract functions.
- Do not copy iterator expression, instead change the type in all
  references to the iterator variable.
- Fix some style issues.

 #KT-47171 Fixed

compiler/testData/codegen/box/controlStructures/forInIterator/abstractNext.kt

@@ -0,0 +1,24 @@
+// TARGET_BACKEND: JVM
+// WITH_STDLIB
+
+fun iterate(iterator: MyIterator): String {
+    for (x in iterator) return x
+    return "Fail"
+}
+
+abstract class MyIterator : Iterator<String> {
+    override fun hasNext(): Boolean = true
+    abstract override fun next(): String
+}
+
+class MyIteratorImpl : MyIterator() {
+    override fun next(): String = "OK"
+}
+
+fun box(): String = iterate(MyIteratorImpl())
+
+// CHECK_BYTECODE_TEXT
+// JVM_IR_TEMPLATES
+// 0 java/util/Iterator.next
+// 1 MyIterator.next
+// 1 MyIteratorImpl.next

compiler/testData/codegen/box/controlStructures/forInIterator/primitiveIterator.kt

@@ -0,0 +1,24 @@
+// TARGET_BACKEND: JVM
+// WITH_STDLIB
+
+fun myFun(iterator: MyLongIterator) {
+    for (x in iterator) {
+        assert(x == 42L)
+    }
+}
+
+class MyLongIterator : Iterator<Long> {
+    private var count = 1
+    override fun hasNext(): Boolean = count-- > 0
+    override fun next(): Long = 42L
+}
+
+fun box(): String {
+    myFun(MyLongIterator())
+    return "OK"
+}
+
+// CHECK_BYTECODE_TEXT
+// JVM_IR_TEMPLATES
+// 0 java/util/Iterator.next
+// 2 MyLongIterator.next

compiler/testData/codegen/box/controlStructures/forInIterator/uintIterator.kt

@@ -0,0 +1,27 @@
+// TARGET_BACKEND: JVM
+// WITH_STDLIB
+
+fun myFun(iterator: MyUIntIterator) {
+    for (x in iterator) {
+        assert(x == 42u)
+    }
+}
+
+class MyUIntIterator : Iterator<UInt> {
+    private var count = 1
+    override fun hasNext(): Boolean = count-- > 0
+    override fun next(): UInt = 42u
+}
+
+fun box(): String {
+    myFun(MyUIntIterator())
+    return "OK"
+}
+
+// CHECK_BYTECODE_TEXT
+// JVM_IR_TEMPLATES
+// 0 java/util/Iterator.hasNext \(\)Z
+// 0 java/util/Iterator.next \(\)Ljava/lang/Object;
+// 1 MyUIntIterator.hasNext \(\)Z
+// 1 public synthetic bridge next\(\)Ljava/lang/Object;
+// 2 MyUIntIterator.next-pVg5ArA \(\)I

compiler/testData/codegen/box/controlStructures/forInIterator/unrelatedExtensionFunctionNext.kt

@@ -0,0 +1,22 @@
+// TARGET_BACKEND: JVM
+// WITH_STDLIB
+
+fun iterate(iterator: MyIterator): String {
+    for (x in iterator) return x
+    return "Fail"
+}
+
+class MyIterator : Iterator<String> {
+    @JvmName("unrelated")
+    private fun String.next(): String = throw AssertionError("Should not be called")
+
+    override fun hasNext(): Boolean = true
+    override fun next(): String = "OK"
+}
+
+fun box(): String = iterate(MyIterator())
+
+// CHECK_BYTECODE_TEXT
+// JVM_IR_TEMPLATES
+// 0 java/util/Iterator.next
+// 2 MyIterator.next