RFR: 8287697: Limit auto vectorization to 32-byte vector on Cascade Lake

[Sandhya Viswanathan]

We observe ~20% regression in SPECjvm2008 mpegaudio sub benchmark on Cascade Lake with Default vs -XX:UseAVX=2.
The performance of all the other non-startup sub benchmarks of SPECjvm2008 is within +/- 5%. 
The performance regression is due to auto-vectorization of small loops. 
We don’t have AVX3Threshold consideration in auto-vectorization. 
The performance regression in mpegaudio can be recovered by limiting auto-vectorization to 32-byte vectors.

This PR limits auto-vectorization to 32-byte vectors by default on Cascade Lake. Users can override this by either setting -XX:UseAVX=3 or -XX:SuperWordMaxVectorSize=64 on JVM command line.

Please review.

Best Regard,
Sandhya

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Commit messages:
 - x86 build fix
 - Fix 32-bit build
 - review comment resolution
 - Change option name and add checks
 - Limit auto vectorization to 32 byte vector on Cascade Lake

Changes: https://git.openjdk.java.net/jdk/pull/8877/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8877&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287697
  Stats: 53 lines in 6 files changed: 45 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8877.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8877/head:pull/8877

PR: https://git.openjdk.java.net/jdk/pull/8877