[vectorIntrinsics] RFR: 8283709: Add x86 back-end implementation for bit BIT_COUNT operation

[Jatin Bhateja]

Summary of changes:

- Patch re-uses existing C2 IR nodes and re-organizes LUT based JIT code sequence of VectorOperations.BIT_COUNT operation 
  for sub-word type (BYTE, SHORT) vectors over X86 targets supporting AVA2 and AVX512 features.
- Efficient single instruction POPCOUNT instruction is emitted for applicable targets. 

Kindly review and share you feedback.

Best Regards,
Jatin

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Commit messages:
 - 8283709: Updating PopCountVI instruction size cost model.
 - 8283709: Add x86 back-end implementation for bit BIT_COUNT operation

Changes: https://git.openjdk.java.net/panama-vector/pull/185/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-vector&pr=185&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283709
  Stats: 156 lines in 10 files changed: 95 ins; 18 del; 43 mod
  Patch: https://git.openjdk.java.net/panama-vector/pull/185.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-vector pull/185/head:pull/185

PR: https://git.openjdk.java.net/panama-vector/pull/185