KT-234 Force when() expressions to have an 'else' branch

KT-973 Unreachable code

compiler/testData/diagnostics/tests/Super.jet

@@ -6,7 +6,7 @@ fun notAnExpression() {
     any(<!SUPER_IS_NOT_AN_EXPRESSION!>super<!>) // not an expression
     if (<!SUPER_IS_NOT_AN_EXPRESSION!>super<!>) {} else {} // not an expression
     val <!UNUSED_VARIABLE!>x<!> = <!SUPER_IS_NOT_AN_EXPRESSION!>super<!> // not an expression
-    when (1) {
+    <!NO_ELSE_IN_WHEN!>when<!> (1) {
         <!SUPER_IS_NOT_AN_EXPRESSION!>super<!> -> 1 // not an expression
     }
 
@@ -86,4 +86,4 @@ class A1 {
     fun test() {
         <!SUPER_IS_NOT_AN_EXPRESSION!>super<!>.equals(null)
     }
-}
+}