RFR: 8252583: Clean up unused phi-to-copy degradation mechanism [v4]

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

  Clean up unused PhiNode-to-copy degradation
  
  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().

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/275/files
  - new: https://git.openjdk.java.net/jdk/pull/275/files/844a31af..91f30bae

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=275&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=275&range=02-03

  Stats: 27 lines in 7 files changed: 1 ins; 22 del; 4 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