[vectorIntrinsics] RFR: Optimize mem barriers for ByteBuffer cases [v6]

[Paul Sandoz]

On Tue, 3 Aug 2021 18:54:24 GMT, Radoslaw Smogura <github.com+7535718+rsmogura at openjdk.org> wrote:

> Hi all, I think that for this PR, I rather don't want to push more changes. It already stepped out from the original concern of optimizing mem-bars, to optimizing polluted cases.

Yeah, I am inclined to dial back to focus this PR on just the non-polluted cases. 
AFAICT I think we can get a set of minimal changes using casting and unsafe for accessing buffer state with no need to modify byte buffer code or branch on heao/direct. WDYT?

We can then expand later with further experiments for polluted cases, evaluating the costs and benefits.

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PR: https://git.openjdk.java.net/panama-vector/pull/104