RFR: 8292368: [ppc64] internal error g1BarrierSet.inline.hpp assert(oopDesc::is_oop(pre_val, true)) failed: Error

[Martin Doerr]

C1 patching stubs contain null Oops on PPC64. Null check is missing in [JDK-8292077](https://bugs.openjdk.org/browse/JDK-8292077).

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Commit messages:
 - 8292368: [ppc64] internal error g1BarrierSet.inline.hpp assert(oopDesc::is_oop(pre_val, true)) failed: Error

Changes: https://git.openjdk.org/jdk/pull/9968/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9968&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292368
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/9968.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9968/head:pull/9968

PR: https://git.openjdk.org/jdk/pull/9968