Split ReplaceSingleLineLetInspection to SimpleRedundantLet & ComplexRedundantLet

Relates to #KT-32010

idea/testData/inspectionsLocal/complexRedundantLet/assignment.kt

@@ -0,0 +1,8 @@
+// PROBLEM: none
+// WITH_RUNTIME
+
+fun withAssign(arg: String?): String {
+    var result: String = ""
+    arg?.let<caret> { result = it }
+    return result
+}