[vectorIntrinsics] RFR: 8266286: Add LoadVectorGather and StoreVectorScatter's partial version [v2]

[Wang Huang]

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

Wang Huang has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR.

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Changes:
  - all: https://git.openjdk.java.net/panama-vector/pull/87/files
  - new: https://git.openjdk.java.net/panama-vector/pull/87/files/7a87daf9..5f57f548

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=panama-vector&pr=87&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=panama-vector&pr=87&range=00-01

  Stats: 192 lines in 2 files changed: 8 ins; 184 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