[vectorIntrinsics] RFR: 8265817: Add LoadVectorGather nodes implementation for Arm SVE

[Wang Huang]

* Add LoadVectorGather nodes implementation for Arm SVE
* add new instruction `sve_ld1w_gather`[1] & `sve_ld1d_gather`[2]
* move a small code into `op_sve_supported` as @nsjian suggested[3]
* some code styles fix 
[1] https://developer.arm.com/documentation/ddi0596/2020-12/SVE-Instructions/LD1W--scalar-plus-vector---Gather-load-unsigned-words-to-vector--vector-index--
[2] https://developer.arm.com/documentation/ddi0596/2020-12/SVE-Instructions/LD1D--scalar-plus-vector---Gather-load-doublewords-to-vector--vector-index--
[3]https://github.com/openjdk/panama-vector/pull/70#discussion_r618240204

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Commit messages:
 - 8265817: Add LoadVectorGather nodes implementation for Arm SVE

Changes: https://git.openjdk.java.net/panama-vector/pull/72/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-vector&pr=72&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265817
  Stats: 181 lines in 6 files changed: 93 ins; 16 del; 72 mod
  Patch: https://git.openjdk.java.net/panama-vector/pull/72.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-vector pull/72/head:pull/72

PR: https://git.openjdk.java.net/panama-vector/pull/72