[vectorIntrinsics] RFR: 8284459: Add x86 back-end implementation for LEADING_ZEROS_COUNT operation [v2]

[Jatin Bhateja]

> Summary of changes:
> - Patch extends SLP vectorizer to auto-vectorize Integer.numberOfLeadingZeros() and Long.numberOfLeadingZeros() APIs.
> - Adds optimized target feature specific X86 backend implementation for LEADING_ZEROS_COUNT operation for all integral types. 
> 
> Kindly review and share your feedback.
> 
> Best Regards,
> Jatin

Jatin Bhateja has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains three commits:

 - 8284459: Adding an exponent based leading zero count algorithm for integer vectors, its showing around 10-15% gain.
 - Merge branch 'vectorIntrinsics' of http://github.com/openjdk/panama-vector into JDK-8284459
 - 8284459: Add x86 back-end implementation for LEADING_ZERO_COUNT operation

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Changes: https://git.openjdk.java.net/panama-vector/pull/189/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-vector&pr=189&range=01
  Stats: 426 lines in 14 files changed: 414 ins; 4 del; 8 mod
  Patch: https://git.openjdk.java.net/panama-vector/pull/189.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-vector pull/189/head:pull/189

PR: https://git.openjdk.java.net/panama-vector/pull/189