RFR: 8289856: [PPC64] SIGSEGV in C2Compiler::init_c2_runtime() after JDK-8289060

[Martin Doerr]

We're currently calling `nullptr->is_valid()` and `nullptr->value()` which causes SIGSEGV on PPC64 (and is undefined behavior). See JBS for details.

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Commit messages:
 - 8289856: [PPC64] SIGSEGV in C2Compiler::init_c2_runtime() after JDK-8289060

Changes: https://git.openjdk.org/jdk/pull/9403/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9403&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289856
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/9403.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9403/head:pull/9403

PR: https://git.openjdk.org/jdk/pull/9403