RFR: 8289856: [PPC64] SIGSEGV in C2Compiler::init_c2_runtime() after JDK-8289060 [v2]

[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.

Martin Doerr has updated the pull request incrementally with one additional commit since the last revision:

  Use VMRegImpl::Bad() for vector registers and remove null check again.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/9403/files
  - new: https://git.openjdk.org/jdk/pull/9403/files/15ca35ae..58cefd26

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9403&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9403&range=00-01

  Stats: 66 lines in 2 files changed: 0 ins; 0 del; 66 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