RFR: 6232281: -XX:-UseLoopSafepoints causes assert(v_false, "Parse::remove_useless_nodes missed this node") [v2]

[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.

Roberto Castañeda Lozano has updated the pull request incrementally with two additional commits since the last revision:

 - Remove redundant -Xbatch option in test case
 - Remove confusing comment in test case

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1311/files
  - new: https://git.openjdk.java.net/jdk/pull/1311/files/5074f818..4129529c

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1311&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1311&range=00-01

  Stats: 4 lines in 1 file changed: 0 ins; 2 del; 2 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