"Introduce 'when' subject" intention: suggest for "this" expression

#KT-12567 Fixed

idea/testData/inspectionsLocal/branched/introduceWhenSubject/this.kt

@@ -0,0 +1,7 @@
+fun Number.test() {
+    <caret>when {
+        this is Int -> {}
+        this is Long -> {}
+        this is Short -> {}
+    }
+}

idea/testData/inspectionsLocal/branched/introduceWhenSubject/this.kt.after

@@ -0,0 +1,7 @@
+fun Number.test() {
+    <caret>when (this) {
+        is Int -> {}
+        is Long -> {}
+        is Short -> {}
+    }
+}