RFR: JDK-8297968: Crash in PrintOptoAssembly

[Andrew Haley]

If PrintOptoAssembly is used in an optimized build, we have a crash in `PhaseChaitin::dump_frame()` due to reading from uninitialized memory in the _parm_regs array. The fix is trivial.

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Commit messages:
 - JDK-8297968: Crash in PrintOptoAssembly

Changes: https://git.openjdk.org/jdk/pull/11460/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11460&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297968
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11460.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11460/head:pull/11460

PR: https://git.openjdk.org/jdk/pull/11460