[vector] RFR 8218750: Vectorized ChaCha20 Benchmark

[Adam Petcher]

Webrev: https://cr.openjdk.java.net/~apetcher/8218750/webrev.00/
JBS: https://bugs.openjdk.java.net/browse/JDK-8218750

This is a benchmark based on an implementation of ChaCha20[1] on the 
vector API. I added the benchmark to the existing microbenchmark project 
for jdk.incubator.vector, but it is in a separate top-level package so 
it doesn't interfere with the generated code. Let me know if I should 
reorganize this. I've included the result of running this benchmark and 
the existing (scalar) ChaCha20 benchmark on my Haswell laptop. The 
128-bit vectorized implementation is a bit faster than the scalar 
implementation, and at 256 bits it is over twice as fast.

[1] https://eprint.iacr.org/2013/759.pdf


Benchmark                     (dataSize)  (keyLength)  (mode)  (padding)  (permutation)  (provider)   Mode  Cnt      Score     Error  Units
CipherBench.ChaCha20.encrypt       16384          256    None  NoPadding       ChaCha20              thrpt   40  26890.258 ± 196.494  ops/s

Benchmark            (dataSize)  (vectorWidth)   Mode  Cnt      Score     Error  Units
ChaChaBench.encrypt       16384            128  thrpt    8  34560.979 ± 344.393  ops/s
ChaChaBench.encrypt       16384            256  thrpt    8  60696.328 ± 587.721  ops/s
ChaChaBench.encrypt       16384            512  thrpt    8   1148.408 ±  27.487  ops/s