Filter return Unit from tail-optimized suspend funs

[poseidon-mysugr]

No description provided.

[ChristopherKoellner]

Could you also attach the bytecode to this pull request?
Others than that LGTM!

[poseidon-mysugr]

This function ignores some uncoverable instructions and branchs of suspend funs:

    suspend fun example(b: Boolean) {
        if (b)
            suspendingFunction()
        else
            suspendingFunction()
    }

produces the following bytecode:

   L0
    ILOAD 1
    IFEQ L1
   L2
    ALOAD 0
    ALOAD 2
    INVOKEVIRTUAL suspendingFunction (Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
>   DUP
>   INVOKESTATIC kotlin/coroutines/intrinsics/IntrinsicsKt.getCOROUTINE_SUSPENDED ()Ljava/lang/Object;
>   IF_ACMPNE L3
>   ARETURN
>  L3
>   POP
    GOTO L4
   L1
    ALOAD 0
    ALOAD 2
    INVOKEVIRTUAL suspendingFunction (Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
>   DUP
>   INVOKESTATIC kotlin/coroutines/intrinsics/IntrinsicsKt.getCOROUTINE_SUSPENDED ()Ljava/lang/Object;
>   IF_ACMPNE L5
>   ARETURN
>  L5
>   POP
   L4
>   GETSTATIC kotlin/Unit.INSTANCE : Lkotlin/Unit;
>   ARETURN

Here, the first two blocks marked with > need to be ignored, as they are only executed if the coroutine function call (suspendingFunction) actually does suspend, in which case the COROUTINE_SUSPENDED marker is passed on. The last block at the bottom in turn cannot be covered if the suspendingFunction never suspends.