RFR: 8252583: Make PhiNode::is_copy() debug only [v3]

[Roberto Castañeda Lozano]

> Convert `PhiNode::is_copy()` into an actual, debug-only predicate. Replace calls to `is_copy()` from non-debug code
> with explicit assertions. Remove dead loop in debug-only `MergeMemStream::match_memory()`.

Roberto Castañeda Lozano has updated the pull request with a new target base due to a merge or a rebase. The
incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four
additional commits since the last revision:

 - Merge branch 'master' of github.com:robcasloz/jdk into JDK-8252583
 - Simplify assertions by folding in phi tests
 - Fix spacing in touched lines
 - 8252583: Make PhiNode::is_copy() debug only
   
   Convert PhiNode::is_copy() into an actual, debug-only predicate. Replace calls
   to is_copy() from non-debug code with explicit assertions. Remove dead loop in
   debug-only MergeMemStream::match_memory().

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/275/files
  - new: https://git.openjdk.java.net/jdk/pull/275/files/53962988..844a31af

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=275&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=275&range=01-02

  Stats: 7965 lines in 297 files changed: 4684 ins; 2611 del; 670 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