RFR: 8259278: Optimize Vector API slice and unslice operations

[Sandhya Viswanathan]

This pull request optimizes Vector API slice and unslice operations.
All the slice and unslice variants that take more than one argument are implemented in terms of already intrinsic methods on similar lines as slice(origin) and unslice(origin).
Also, the slice and unslice intrinsics for 256-bit byte/short vectors implemented for x86 platforms supporting AVX2 using a sequence of instructions.

For TestSlice.java attached to JBS (https://bugs.openjdk.java.net/browse/JDK-8259278):
Before:
Benchmark                                                       (size) Mode  Cnt    Score    Error   Units
TestSlice.vectorSliceOrigin                               1024  thrpt    5    17.665 ±  0.980  ops/ms
TestSlice.vectorSliceOriginVector                     1024  thrpt    5  604.179 ±  5.795  ops/ms
TestSlice.vectorSliceUnsliceOrigin                    1024  thrpt    5      9.286 ±  0.088  ops/ms
TestSlice.vectorSliceUnsliceOriginVector          1024  thrpt    5  435.678 ± 30.171 ops/ms
TestSlice.vectorSliceUnsliceOriginVectorPart    1024  thrpt    5  440.492 ± 24.592 ops/ms

After:
Benchmark                                                        (size)   Mode  Cnt     Score    Error   Units
TestSlice.vectorSliceOrigin                                1024  thrpt    5  1319.044 ± 67.862  ops/ms
TestSlice.vectorSliceOriginVector                      1024  thrpt    5    969.514 ± 33.411  ops/ms
TestSlice.vectorSliceUnsliceOrigin                     1024  thrpt    5    687.804 ± 31.511  ops/ms
TestSlice.vectorSliceUnsliceOriginVector           1024  thrpt    5    560.807 ± 20.600  ops/ms
TestSlice.vectorSliceUnsliceOriginVectorPart     1024  thrpt    5    560.202 ±  4.012  ops/ms

Please review.

Best Regards,
Sandhya

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Commit messages:
 - 8259278: Optimize Vector API slice and unslice operations

Changes: https://git.openjdk.java.net/jdk/pull/1950/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1950&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259278
  Stats: 1475 lines in 45 files changed: 563 ins; 285 del; 627 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1950.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1950/head:pull/1950

PR: https://git.openjdk.java.net/jdk/pull/1950