Added test and asserts

idea/testData/intentions/loopToCallChain/filterIndexed_merge4.kt

@@ -0,0 +1,11 @@
+// WITH_RUNTIME
+// INTENTION_TEXT: "Replace with 'filterIndexed{}.firstOrNull()'"
+fun foo(list: List<String>): String? {
+    var index2 = 0
+    <caret>for ((index1, s) in list.withIndex()) {
+        if (s.length > index1) continue
+        if (s.length < index2++) continue
+        return s
+    }
+    return null
+}

idea/testData/intentions/loopToCallChain/filterIndexed_merge4.kt.after

@@ -0,0 +1,8 @@
+// WITH_RUNTIME
+// INTENTION_TEXT: "Replace with 'filterIndexed{}.firstOrNull()'"
+fun foo(list: List<String>): String? {
+    var index2 = 0
+    return list
+            .filterIndexed { index1, s -> s.length <= index1 && s.length >= index2++ }
+            .firstOrNull()
+}