Allow if/when/try postfix templates in expression position

#KT-14078 Fixed

idea/testData/codeInsight/postfix/whenExpression.kt

@@ -0,0 +1,3 @@
+fun foo(x: String) {
+    val y = x.when<caret>
+}

idea/testData/codeInsight/postfix/whenExpression.kt.after

@@ -0,0 +1,8 @@
+fun foo(x: String) {
+    val y = when (x) {
+         -> {
+        }
+        else -> {
+        }
+    }
+}