RFR: 8266438: Compile::remove_useless_nodes does not remove opaque nodes

[Tobias Hartmann]

[JDK-8255026](https://bugs.openjdk.java.net/browse/JDK-8255026) refactored the code in `Compile::remove_useless_nodes` and as a result, useless nodes are no longer removed from the `_predicate_opaqs` list. Before the [change](https://github.com/openjdk/jdk/commit/27230fae#diff-f076857d7da81f56709da3de1511b1105727032186cde4d02c678667761f46eaL382), the call to `remove_macro_node` took care of this:
https://github.com/openjdk/jdk/blob/194bceca3a4d13d4528b86359ee9d5eead3ce7ac/src/hotspot/share/opto/compile.hpp#L676-L684

But the new code only removes nodes from the `_macro_nodes` list. Useless nodes should be removed from the `_skeleton_predicate_opaqs` list as well.

I've seen failures due to this with a change in Valhalla (where we call `remove_useless_nodes` more often) but not in mainline. I think this should still be fixed in mainline.

Thanks,
Tobias

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Commit messages:
 - 8266438: Compile::remove_useless_nodes does not remove opaque nodes

Changes: https://git.openjdk.java.net/jdk/pull/3840/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3840&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266438
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3840.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3840/head:pull/3840

PR: https://git.openjdk.java.net/jdk/pull/3840