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

[Xin Liu]

On Sun, 11 Oct 2020 21:49:37 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 filed JDK-8254369 for the bug.  Let's keep this PR as a clean-up.

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