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/testData/codegen/bytecodeListing/tailcall/whenUnit.kt

@@ -0,0 +1,12 @@
+sealed class X {
+    class A : X()
+    class B : X()
+}
+
+suspend fun process(a: X.A) {}
+suspend fun process(b: X.B) {}
+
+suspend fun process(x: X) = when (x) {
+    is X.A -> process(x)
+    is X.B -> process(x)
+}