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

[Xiaohong Gong]

On Wed, 17 Mar 2021 01:29:47 GMT, Wang Huang <whuang at openjdk.org> wrote:

> 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 use  `rscratch1`

LGTM, thanks!

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