[vectorIntrinsics] RFR: Add utf8 decoding benchmarks

[Ludovic Henry]

On Tue, 24 Nov 2020 00:46:37 GMT, Ludovic Henry <luhenry at openjdk.org> wrote:

> Following discussions on the mailing list, I'm submitting three benchmarks around UTF-8 decoding:
>  - decode: uses a while-loop based implementation currently in use in the JDK
>  - decodeVector: uses a lookup table with vector operations for 1-3 bytes characters
>  - decodeVectorASCII: uses a simple vector operation to accelerate parsing ASCII-only characters
> 
> We don't observe the expected speedups with either decodeVector and decodeVectorASCII, so these are, I think, good test cases to further develop the Vector API.

> @luhenry Looks good to me as well. Please do create a separate branch (other than vectorIntrinsics) as indicated above by OpenJDK Skara bot and resubmit the pull request from the new branch.

@sviswa7 the only issue it introduces is that, once this PR is merged, I'll need to force-push to my [luhenry/panama-vector:vectorIntrinsics](https://github.com/luhenry/panama-vector/commits/vectorIntrinsics) branch as it will have diverged from [openjdk/panama-vector:vectorIntrinsics](https://github.com/openjdk/panama-vector/commits/vectorIntrinsics) branch. But that is something I'm used to working with, and thus shouldn't be a blocker for this PR.

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PR: https://git.openjdk.java.net/panama-vector/pull/26