RFR: 8253748: StressIGV tests fail with release VMs

[Jie Fu]

The following StressIGV tests fails with release VMs
  - compiler/arguments/TestStressIGVNOptions.java
  - compiler/debug/TestGenerateStressSeed.java
  - compiler/debug/TestStressIGVN.java

The reason is that VM option 'StressIGVN' is diagnostic and must be enabled via -XX:+UnlockDiagnosticVMOptions.

The fix adds '-XX:+UnlockDiagnosticVMOptions' option.
And compiler/debug/TestStressIGVN.java should be only available for debug VMs since it depends
on -XX:+TraceIterativeGVN.

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Commit messages:
 - 8253748: StressIGV tests fail with release VMs

Changes: https://git.openjdk.java.net/jdk/pull/390/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=390&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253748
  Stats: 4 lines in 3 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/390.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/390/head:pull/390

PR: https://git.openjdk.java.net/jdk/pull/390