RFR: 8278868: Add x86 vectorization support for Long.bitCount() [v4]

[Jatin Bhateja]

On Wed, 5 Jan 2022 04:04:03 GMT, Vamsi Parasa <duke at openjdk.java.net> wrote:

>> 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(). This patch shows 2.57x improvement in performance on a JMH micro benchmark due to x86 vectorization.
>
> Vamsi Parasa has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update popcount long test to use IR framework

Please also update copywrite headers of modified files.

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