RFR: 8261553: Efficient mask generation using BMI2 BZHI instruction

[Jatin Bhateja]

BMI2 BHZI instruction can be used to optimize the instruction sequence
used for mask generation at various place in array copy stubs and partial in-lining for small copy operations.

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Commit messages:
 - 8261553: Efficient mask generation using BMI2 BZHI instruction.

Changes: https://git.openjdk.java.net/jdk/pull/2522/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2522&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261553
  Stats: 30 lines in 5 files changed: 8 ins; 11 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2522.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2522/head:pull/2522

PR: https://git.openjdk.java.net/jdk/pull/2522