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

[Roberto Castañeda Lozano]

On Tue, 22 Sep 2020 12:54:31 GMT, Roberto Castañeda Lozano <github.com+8792647+robcasloz at openjdk.org> wrote:

>> Reverted to "draft mode", as I just realized the design is not repeatable since it relies on global PRNG state.
>
> 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 new options are declared as production+diagnostic for consistency with the
> existing 'StressLCM' and 'StressGCM' options.

This pull request is ready for review again.

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