RFR: 8277155: Compress and expand vector operations

[Paul Sandoz]

Add two new cross-lane vector operations, `compress` and `expand`.

An example of such usage might be code that selects elements from array `a` and stores those selected elements in array `z`:


int[] a = ...;

int[] z = ...;
int ai = 0, zi = 0;
while (ai < a.length) {
    IntVector av = IntVector.fromArray(SPECIES, a, ai);
    // query over elements of vector av
    // returning a mask marking elements of interest
    VectorMask<Integer> m = interestingBits(av, ...);
    IntVector zv = av.compress(m);
    zv.intoArray(z, zi, m.compress());
    ai += SPECIES.length();
    zi += m.trueCount();
}


(There's also a more sophisticated version using `unslice` to coalesce matching elements with non-masked stores.)

Given RDP 1 for 18 is getting close, 2021/12/09, we may not get this reviewed in time and included in [JEP 417](https://openjdk.java.net/jeps/417). Still I think I think it worth starting the review now (the CSR is marked provisional).

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Commit messages:
 - 8277155: Compress and expand vector operations

Changes: https://git.openjdk.java.net/jdk/pull/6545/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6545&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277155
  Stats: 5429 lines in 105 files changed: 5315 ins; 21 del; 93 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6545.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6545/head:pull/6545

PR: https://git.openjdk.java.net/jdk/pull/6545