RFR: 8252219: C2: Randomize IGVN worklist for stress testing

Fri Sep 18 10:57:44 UTC 2020

Add `StressIGVN` option to let C2 randomize IGVN worklist order. When enabled, the worklist is shuffled before each
main run of the IGVN loop. Also add `GenerateStressSeed' and 'StressSeed=N` options to randomly generate or specify the
seed. In either case, the seed is logged if `LogCompilation` is enabled.

The generation or specification of seeds also affects the randomization triggered by `StressLCM` and `StressGCM`. The
new options are declared as production+diagnostic for consistency with these existing options.

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Commit messages:
 - 8252219: C2: Randomize IGVN worklist for stress testing

Changes: https://git.openjdk.java.net/jdk/pull/242/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=242&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252219
  Stats: 172 lines in 7 files changed: 172 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/242.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/242/head:pull/242

PR: https://git.openjdk.java.net/jdk/pull/242