RFR: 8252583: Make PhiNode::is_copy() debug only

[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()`.

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Commit messages:
 - 8252583: Make PhiNode::is_copy() debug only

Changes: https://git.openjdk.java.net/jdk/pull/275/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=275&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252583
  Stats: 32 lines in 7 files changed: 15 ins; 4 del; 13 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