RFR: 8252583: Make PhiNode::is_copy() debug only [v2]

[Xin Liu]

On Mon, 21 Sep 2020 20:52:19 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

> Sorry, I am not sure this is better than original code where assert is in one place - in PhiNode::is_copy().
> The method is in .hpp file and is inlined - NULL check will be eliminated. It will only be executed in slowdebug build.
> May be we should "bite the bullet" and remove this method at all - we don't hit the assert in years.
> We can replace is_copy() check with assert in PhiNode::Ideal() - that should be enough to guarantee that we don't
> create Phi with NULL control edge.

Another thing is that  a node has a member called uint Node::is_Copy() const.  PhiNode/Region nodes both have a member
callled "is_copy()".  Is it intentional?  IMHO, it's not good in style.

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PR: https://git.openjdk.java.net/jdk/pull/275