Introduce "cascade if" inspection suggesting replacement with when

So #KT-18615 Fixed

idea/testData/inspectionsLocal/cascadeIf/ifTree.kt

@@ -0,0 +1,16 @@
+// PROBLEM: none
+
+fun println(s: String) {}
+
+fun foo(a: Boolean, b: Boolean) {
+    <caret>if (a) {
+        if (b) println("ab")
+        else println("a")
+    }
+    else if (b) {
+        println("b")
+    }
+    else {
+        println("none")
+    }
+}