psi2ir: Fix return insertion

We should only insert a return statement at the end of a lambda or
function if the final statement is used as an expression (slice
USED_AS_RESULT_OF_LAMBDA and USED_AS_EXPRESSION).

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

@@ -1286,6 +1286,11 @@ public class IrTextTestCaseGenerated extends AbstractIrTextTestCase {
             runTest("compiler/testData/ir/irText/expressions/whenReturn.kt");
         }
 
+        @TestMetadata("whenReturnUnit.kt")
+        public void testWhenReturnUnit() throws Exception {
+            runTest("compiler/testData/ir/irText/expressions/whenReturnUnit.kt");
+        }
+
         @TestMetadata("whenUnusedExpression.kt")
         public void testWhenUnusedExpression() throws Exception {
             runTest("compiler/testData/ir/irText/expressions/whenUnusedExpression.kt");