RFR: 8336702: C2 compilation fails with "all memory state should have been processed" assert

[Roland Westrelin]

When converting a `LongCountedLoop` into a loop nest, c2 needs jvm
state to add predicates to the inner loop. For that, it peels an
iteration of the loop and uses the state of the safepoint at the end
of the loop. That's only legal if there's no side effect between the
safepoint and the backedge that goes back into the loop. The assert
failure here happens in code that checks that.

That code compares the memory states at the safepoint and at the
backedge. If they are the same then there's no side effect. To check
consistency, the `MergeMem` at the safepoint is cloned. As the logic
iterates over the backedge state, it clears every component of the
state it encounters from the `MergeMem`. Once done, the cloned
`MergeMem` should be "empty". In the case of this failure, no side
effect is found but the cloned `MergeMem` is not empty. That happens
because of EA: it adds edges to the `MergeMem` at the safepoint that
it doesn't add to the backedge `Phis`.

So it's the verification code that fails and I propose dealing with
this by ignoring memory state added by EA in the verification code.

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Commit messages:
 - fix & test

Changes: https://git.openjdk.org/jdk/pull/21009/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21009&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336702
  Stats: 71 lines in 2 files changed: 69 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/21009.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21009/head:pull/21009

PR: https://git.openjdk.org/jdk/pull/21009