[vectorIntrinsics] RFR: 8283709: Add x86 back-end implementation for bit BIT_COUNT operation [v3]

[Ningsheng Jian]

On Mon, 28 Mar 2022 05:23:49 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

>> Is it possible to separate this cast for SLP and always uses "T_LONG' as the basic type for `PopCountVL` ?
>
> We can have type homogeneity for PopCountVL across VectorAPI and SLP once we start auto-vectorizing ConvL2I  operation.

I think that makes the code hard to maintain, and I think @merykitty 's unaddressed comment in: https://github.com/openjdk/jdk/pull/6857#discussion_r780917717 is a preferable approach. 

FYI: @fg1417 has a ConvL2I auto-vectorization support under review: https://github.com/openjdk/jdk/pull/7806.

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