Introduce "cascade if" inspection suggesting replacement with when

So #KT-18615 Fixed

idea/testData/inspectionsLocal/cascadeIf/.inspection

@@ -0,0 +1 @@
+org.jetbrains.kotlin.idea.inspections.CascadeIfInspection

idea/testData/inspectionsLocal/cascadeIf/four.kt

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

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")
+    }
+}

idea/testData/inspectionsLocal/cascadeIf/insideOtherIf.kt

@@ -0,0 +1,15 @@
+fun println(s: String) {}
+
+fun test(x: Double, a: Boolean, b: Boolean) {
+    if (x > 0.0) {
+        <caret>if (a) {
+            println("a")
+        }
+        else if (b) {
+            println("b")
+        }
+        else {
+            println("none")
+        }
+    }
+}

idea/testData/inspectionsLocal/cascadeIf/insideOtherIf.kt.after

@@ -0,0 +1,11 @@
+fun println(s: String) {}
+
+fun test(x: Double, a: Boolean, b: Boolean) {
+    if (x > 0.0) {
+        when {
+            a -> println("a")
+            b -> println("b")
+            else -> println("none")
+        }
+    }
+}

idea/testData/inspectionsLocal/cascadeIf/middleIf.kt

@@ -0,0 +1,15 @@
+// PROBLEM: none
+
+fun println(s: String) {}
+
+fun test(a: Boolean, b: Boolean) {
+    if (a) {
+        println("a")
+    }
+    else <caret>if (b) {
+        println("b")
+    }
+    else {
+        println("none")
+    }
+}

idea/testData/inspectionsLocal/cascadeIf/noSecondElse.kt

@@ -0,0 +1,12 @@
+// PROBLEM: none
+
+fun println(s: String) {}
+
+fun test(a: Boolean, b: Boolean) {
+    <caret>if (a) {
+        println("a")
+    }
+    else if (b) {
+        println("b")
+    }
+}

idea/testData/inspectionsLocal/cascadeIf/normal.kt

@@ -0,0 +1,13 @@
+fun println(s: String) {}
+
+fun test(a: Boolean, b: Boolean) {
+    <caret>if (a) {
+        println("a")
+    }
+    else if (b) {
+        println("b")
+    }
+    else {
+        println("none")
+    }
+}

idea/testData/inspectionsLocal/cascadeIf/normal.kt.after

@@ -0,0 +1,9 @@
+fun println(s: String) {}
+
+fun test(a: Boolean, b: Boolean) {
+    when {
+        a -> println("a")
+        b -> println("b")
+        else -> println("none")
+    }
+}

idea/testData/inspectionsLocal/cascadeIf/shortIf.kt

@@ -0,0 +1,3 @@
+// PROBLEM: none
+
+val x = <caret>if (2 > 4) 1 else -1

idea/testData/inspectionsLocal/cascadeIf/withBreak.kt

@@ -0,0 +1,17 @@
+// PROBLEM: none
+
+fun println(s: String) {}
+
+fun foo(size: Int, a: Boolean, b: Boolean) {
+    for (i in 1..size) {
+        <caret>if (a) {
+            break
+        }
+        else if (b) {
+            println("$i")
+        }
+        else {
+            println("*")
+        }
+    }
+}