RFR: 8291600: [vectorapi] vector cast op check is not always needed for vector mask cast

[Xiaohong Gong]

Recently we found the performance of "`FIRST_NONZERO`" for double type is largely worse than the other types on x86 when `UseAVX=2`. The main reason is the "`VectorCastL2X`" op is not supported by the backend when the dst element type is `T_DOUBLE`. This makes the check of `VectorCast` op fail before intrinsifying "`VectorMask.cast()`" which is used in the
"`FIRST_NONZERO`" java implementation (see [1]). However, the compiler will not generate the `VectorCast `op for `VectorMask.cast()` if:

 1) the current platform supports the predicated feature
 2) the element size (in bytes) of the src and dst type is the same

So the check of "`VectorCast`" op is needless for such cases. To fix it, this patch:

 1) limits the specified vector cast op check to vectors
 2) adds the relative mask cast op check for VectorMask.cast()
 3) cleans up the unnecessary codes

Here is the performance of "`FIRST_NONZERO`" benchmark [2] on a x86 machine with `UseAVX=2`:

Benchmark                          (size) Mode Cnt Before  After   Units
DoubleMaxVector.FIRST_NONZERO       1024  thrpt 15 49.266 2460.886 ops/ms
DoubleMaxVector.FIRST_NONZEROMasked 1024  thrpt 15 49.554 1892.223 ops/ms

[1] https://github.com/openjdk/jdk/blob/master/src/jdk.incubator.vector/share/classes/jdk/incubator/vector/DoubleVector.java#L770
[2] https://github.com/openjdk/panama-vector/blob/vectorIntrinsics/test/micro/org/openjdk/bench/jdk/incubator/vector/operation/DoubleMaxVector.java#L246

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Commit messages:
 - 8291600: [vectorapi] vector cast op check is not always needed for vector mask cast

Changes: https://git.openjdk.org/jdk/pull/9737/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9737&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291600
  Stats: 80 lines in 4 files changed: 46 ins; 8 del; 26 mod
  Patch: https://git.openjdk.org/jdk/pull/9737.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9737/head:pull/9737

PR: https://git.openjdk.org/jdk/pull/9737