RFR: 6232281: -XX:-UseLoopSafepoints causes assert(v_false, "Parse::remove_useless_nodes missed this node")

[Roberto Castañeda Lozano]

Check for nodes missed by `remove_useless_nodes()` only if PhaseRemoveUseless has actually been run. This makes it possible to use `-XX:-UseLoopSafepoints` without crashing trivially, although implicit assumptions in other parts of C2 about the existence of loop safepoints might lead to more subtle failures for more complex methods.

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Commit messages:
 - 6232281: -XX:-UseLoopSafepoints causes assert(v_false,"Parse::remove_useless_nodes missed this node")

Changes: https://git.openjdk.java.net/jdk/pull/1311/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1311&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-6232281
  Stats: 46 lines in 2 files changed: 45 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1311.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1311/head:pull/1311

PR: https://git.openjdk.java.net/jdk/pull/1311