RFR: 8254369: Node::disconnect_inputs may skip precedences

[Xin Liu]

On Wed, 14 Oct 2020 16:04:32 GMT, Xin Liu <xliu at openjdk.org> wrote:

> 8254369: Node::disconnect_inputs may skip precedences

disconnect_inputs() needs to iterate precedences edges in reverse order because rm_prec(i) may backfill _in[i] with a
value afterward.   also remove the predicate if (n != NULL) in set_prec because it's always true.

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PR: https://git.openjdk.java.net/jdk/pull/664