[vectorIntrinsics] RFR: 8263644: Add Extract float nodes implementation for Arm SVE

[Wang Huang]

We implement ExtractD & ExtractF Node for Arm SVE backend.
* overload encoding functions `sve_lasta` & `sve_lastb`
* for future JDK-8263424, we eliminate vector length judgements and replace `tmp` with `rscratch1`

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Commit messages:
 - 8263644: Add Extract float nodes implementation for Arm SVE

Changes: https://git.openjdk.java.net/panama-vector/pull/51/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-vector&pr=51&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263644
  Stats: 119 lines in 5 files changed: 52 ins; 4 del; 63 mod
  Patch: https://git.openjdk.java.net/panama-vector/pull/51.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-vector pull/51/head:pull/51

PR: https://git.openjdk.java.net/panama-vector/pull/51