RFR: 8253721: Flag -XX:AVX3Threshold does not accept Zero value

[Jatin Bhateja]

AVX3Threshold has been used in various places to enable emitting AVX3 specific instructions in case data size being
worked over is greater than 4096 bytes.

However, user is free to set the threshold value to Zero based on his workload.
In such a case a compile time check is enough to trigger generation of AVX3 instructions. In other cases comparison is
done at the runtime though JITed comparison instruction.

Patch allows setting AVX3Threshold to a zero value.

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Commit messages:
 - 8253721: Flag -XX:AVX3Threshold does not accept Zero value.

Changes: https://git.openjdk.java.net/jdk/pull/394/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=394&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253721
  Stats: 23 lines in 7 files changed: 13 ins; 10 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/394.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/394/head:pull/394

PR: https://git.openjdk.java.net/jdk/pull/394