RFR: 8252583: Clean up unused phi-to-copy degradation mechanism [v5]

[Roberto Castaneda Lozano]

> Remove unused notion of "PhiNode-to-copy degradation", where PhiNodes can be degraded to copies by setting their
> RegionNode to NULL. Remove corresponding `PhiNode::is_copy()` test, which always returned NULL (false). Assert that
> PhiNodes have an associated RegionNode in `PhiNode::Ideal()`.

Roberto Castaneda Lozano has updated the pull request incrementally with one additional commit since the last revision:

  Empty commit to trigger jcheck after updating GitHub user name

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/275/files
  - new: https://git.openjdk.java.net/jdk/pull/275/files/91f30bae..b7656e1f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=275&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=275&range=03-04

  Stats: 0 lines in 0 files changed: 0 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/275.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/275/head:pull/275

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