psi2ir: keep the type of when when possible

Ideally, the type of `IrWhen` should be provided by type inference for
a consistent behavior. `USED_AS_EXPRESSION` from CFG isn't always
consistent with type inference, unfortunately.

The behavior is now aligned with `if`. The type of `when` is kept when
it *can* be an expression, instead of whether it is used or not.

compiler/tests/org/jetbrains/kotlin/ir/IrTextTestCaseGenerated.java

@@ -1267,6 +1267,11 @@ public class IrTextTestCaseGenerated extends AbstractIrTextTestCase {
             runTest("compiler/testData/ir/irText/expressions/whenReturn.kt");
         }
 
+        @TestMetadata("whenUnusedExpression.kt")
+        public void testWhenUnusedExpression() throws Exception {
+            runTest("compiler/testData/ir/irText/expressions/whenUnusedExpression.kt");
+        }
+
         @TestMetadata("whenWithSubjectVariable.kt")
         public void testWhenWithSubjectVariable() throws Exception {
             runTest("compiler/testData/ir/irText/expressions/whenWithSubjectVariable.kt");