RFR: 8256986: [PPC64] C2 crashes when accessing nonexisting jvms of CallLeafDirectNode

[Martin Doerr]

observe_safepoint() is called with jvms == NULL from fill_buffer with mach = CallLeafDirectNode.
That node represents a leaf call and does not safepoint.

In addition MachCallRuntimeNode::ret_addr_offset() need update for the new assertion in output.cpp.
This was already fixed on some other platforms.

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Commit messages:
 - 8256986: [PPC64] C2 crashes when accessing nonexisting jvms of CallLeafDirectNode

Changes: https://git.openjdk.java.net/jdk/pull/1418/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1418&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256986
  Stats: 7 lines in 1 file changed: 4 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1418.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1418/head:pull/1418

PR: https://git.openjdk.java.net/jdk/pull/1418