Split ReplaceSingleLineLetInspection to SimpleRedundantLet & ComplexRedundantLet

Relates to #KT-32010

idea/testData/inspectionsLocal/complexRedundantLet/functionCallOnSafeCall.kt

@@ -0,0 +1,9 @@
+// WITH_RUNTIME
+// PROBLEM: none
+
+fun foo(s: String, i: Int) = s.length + i
+
+fun test() {
+    val nullable: String? = null
+    nullable?.let<caret> { foo(it, 1) }
+}