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

[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

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Commit messages:
 - 8284459: Add x86 back-end implementation for LEADING_ZERO_COUNT operation

Changes: https://git.openjdk.java.net/panama-vector/pull/189/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-vector&pr=189&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284459
  Stats: 335 lines in 14 files changed: 323 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