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

native/native.tests/tests-gen/org/jetbrains/kotlin/konan/blackboxtest/NativeCodegenBoxTestGenerated.java

@@ -7261,6 +7261,18 @@ public class NativeCodegenBoxTestGenerated extends AbstractNativeCodegenBoxTest
                 }
             }
 
+            @Nested
+            @TestMetadata("compiler/testData/codegen/box/controlStructures/forInIterator")
+            @TestDataPath("$PROJECT_ROOT")
+            @Tag("codegen")
+            @UseExtTestCaseGroupProvider()
+            public class ForInIterator {
+                @Test
+                public void testAllFilesPresentInForInIterator() throws Exception {
+                    KtTestUtil.assertAllTestsPresentByMetadataWithExcluded(this.getClass(), new File("compiler/testData/codegen/box/controlStructures/forInIterator"), Pattern.compile("^(.+)\\.kt$"), null, TargetBackend.NATIVE, true);
+                }
+            }
+
             @Nested
             @TestMetadata("compiler/testData/codegen/box/controlStructures/forInSequenceWithIndex")
             @TestDataPath("$PROJECT_ROOT")
@@ -14030,18 +14042,6 @@ public class NativeCodegenBoxTestGenerated extends AbstractNativeCodegenBoxTest
             }
         }
 
-        @Nested
-        @TestMetadata("compiler/testData/codegen/box/forLoop")
-        @TestDataPath("$PROJECT_ROOT")
-        @Tag("codegen")
-        @UseExtTestCaseGroupProvider()
-        public class ForLoop {
-            @Test
-            public void testAllFilesPresentInForLoop() throws Exception {
-                KtTestUtil.assertAllTestsPresentByMetadataWithExcluded(this.getClass(), new File("compiler/testData/codegen/box/forLoop"), Pattern.compile("^(.+)\\.kt$"), null, TargetBackend.NATIVE, true);
-            }
-        }
-
         @Nested
         @TestMetadata("compiler/testData/codegen/box/fullJdk")
         @TestDataPath("$PROJECT_ROOT")