[vectorIntrinsics] RFR: 8264579: Use NEON vector load/store to access small size memory

[Ningsheng Jian]

For vector load/store with size 2/4/8/16, we can directly use NEON load/store instead of partial load/store with predicate. Since
currently we are still using different registers (vReg/vecX/vecD) for SVE and NEON codegen, we have to write different rules for NEON and SVE, though they are the same physical regs (sharing lower parts).

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Commit messages:
 - 8264579: Use NEON vector load/store to access small size memory

Changes: https://git.openjdk.java.net/panama-vector/pull/64/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-vector&pr=64&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264579
  Stats: 254 lines in 3 files changed: 194 ins; 12 del; 48 mod
  Patch: https://git.openjdk.java.net/panama-vector/pull/64.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-vector pull/64/head:pull/64

PR: https://git.openjdk.java.net/panama-vector/pull/64