Replace CHECKCAST kotlin/Unit with ARETURN for tail call optimization

Sometimes instead of {POP, GETSTATIC Unit.INSTANCE, ARETURN} sequence
the codegen emits {CHECKCAST Unit, ARETURN} sequence, which breaks tail
call optimization. By replacing CHECKCAST with ARETURN we eliminate
this issue.

 #KT-19790: Fixed

compiler/tests/org/jetbrains/kotlin/codegen/BlackBoxCodegenTestGenerated.java

@@ -6836,6 +6836,12 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
                 String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/box/coroutines/tailCallOptimizations/unreachable.kt");
                 doTest(fileName);
             }
+
+            @TestMetadata("whenUnit.kt")
+            public void testWhenUnit() throws Exception {
+                String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/box/coroutines/tailCallOptimizations/whenUnit.kt");
+                doTest(fileName);
+            }
         }
 
         @TestMetadata("compiler/testData/codegen/box/coroutines/tailOperations")