[vectorIntrinsics] RFR: Add utf8 decoding benchmarks

[Ludovic Henry]

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.

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Commit messages:
 - Remove whitespace
 - Remove some debugging code
 - Add utf8 decoding benchmarks

Changes: https://git.openjdk.java.net/panama-vector/pull/26/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-vector&pr=26&range=00
  Stats: 591 lines in 1 file changed: 591 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/panama-vector/pull/26.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-vector pull/26/head:pull/26

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