RFR: 8254269: simplify Node::disconnect_inputs [v2]

[Xin Liu]

On Sun, 11 Oct 2020 20:06:58 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> Xin Liu has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   8254269: simplify Node::disconnect_inputs
>>   
>>   Optimize it for the precedence loop. because there's no null between
>>   2 non-null precedences, disconnect_inputs can break at a null value.
>
> Changes requested by kvn (Reviewer).

> I think the changes in [b8a7275](https://github.com/openjdk/jdk/commit/b8a72755a29d2fcbe36c9dcaf7f696634f813b4f) might
> be fine, but they are more subtle than the cleanup in the preceding version and I'm not comfortable reviewing them. I
> think it needs a more thorough examination of if those optimizations always hold. Could probably use some more asserts,
> a few tests and lift the documented assumption of the structure of the input to a more visible place. My suggestion is
> to push this ahead with the straightforward cleanup you had in
> [4a9d803](https://github.com/openjdk/jdk/commit/4a9d80306960a1c8da2f95f8578006249acb0a6c) and file a follow-up RFE for
> the other optimization.

yes, thank for the advice.

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PR: https://git.openjdk.java.net/jdk/pull/589