Implement unit suspend functions tail-call optimisation

Unlike previously, this optimisation works on every callee return type.
Tail-calls inside unit functions can be either
INVOKE...
ARETURN
or
INVOKE
POP
GETSTATIC kotlin/Unit.INSTANCE
ARETURN
The first pattern is already covered. The second one is a bit tricky,
since we cannot just assume than the function is tail-call, we also need
to check whether the callee returned COROUTINE_SUSPENDED marker.
Thus, resulting bytecode of function's 'epilogue' look like
DUP
INVOKESTATIC getCOROUTINE_SUSPENDED
IF_ACMPNE LN
ARETURN
LN:
POP

 #KT-28938 Fixed

compiler/testData/codegen/boxInline/suspend/inlineSuspendContinuation.kt

@@ -101,6 +101,6 @@ fun box() : String {
             checkContinuation(savedContinuation!!)
         }
     }
-    if (!continuationChanged) return "FAIL 5"
+    if (continuationChanged) return "FAIL 5"
     return "OK"
 }