[vectorIntrinsics] RFR: 8266286: Add LoadVectorGather and StoreVectorScatter's partial version

[Wang Huang]

* Add LoadVectorGather and StoreVectorScatter's partial version
* use `sve_whilelo`

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Commit messages:
 - 8266286: Add LoadVectorGather and StoreVectorScatter's partial version

Changes: https://git.openjdk.java.net/panama-vector/pull/87/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-vector&pr=87&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266286
  Stats: 192 lines in 2 files changed: 184 ins; 8 del; 0 mod
  Patch: https://git.openjdk.java.net/panama-vector/pull/87.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-vector pull/87/head:pull/87

PR: https://git.openjdk.java.net/panama-vector/pull/87