[vectorIntrinsics+mask] RFR: 8269282: Add masking support for vector gather_load/scatter_store

[Xiaohong Gong]

This patch adds the masking support for masked vector gather load and scatter store, including:
  1) The Vector API java implementation and hotspot intrinsics changes:
    - Add masking information to the `loadWithMap/storeWithMap` intrinsics.
    - Both masked and non-masked gather_load/scatter_store call the same intrinsics. The mask value and class are set to `"null"` for non-masked operations.
  2) The C2 compiler changes:
    - Add two new IRs for the masked gather_load/scatter_store. The new masked IR will be generated for masked operation if the platform supports the predicate feature and the backend has implemented it.  Otherwise, the compiler will go back to the default java implementation.

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Commit messages:
 - 8269282: Add masking support for vector gather_load/scatter_store

Changes: https://git.openjdk.java.net/panama-vector/pull/95/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-vector&pr=95&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269282
  Stats: 1062 lines in 48 files changed: 924 ins; 36 del; 102 mod
  Patch: https://git.openjdk.java.net/panama-vector/pull/95.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-vector pull/95/head:pull/95

PR: https://git.openjdk.java.net/panama-vector/pull/95