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

[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 incrementally with one additional commit since the last revision:

  Fix spacing in touched lines

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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/6175d16a..53962988

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=275&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=275&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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