RFR: 8278868: Add x86 vectorization support for Long.bitCount()

[Vamsi Parasa]

Vectorization support of Integer.bitCount() already exists but currently the same support is lacking for Long.bitCount(). Similar to the C2 PopCountVI node, we created a C2 PopCountVL node and used vpopcntq x86 instruction to enable vectorized Long.bitCount().

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Commit messages:
 - 8278868:Add x86 vectorization support for Long.bitCount()

Changes: https://git.openjdk.java.net/jdk/pull/6857/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6857&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278868
  Stats: 154 lines in 10 files changed: 152 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6857.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6857/head:pull/6857

PR: https://git.openjdk.java.net/jdk/pull/6857