[foreign-memaccess+abi] RFR: Remove mem barriers to allow loop optimizations for vectors accessing off-heap

[Radoslaw Smogura]

This patch is to remove memory barriers, to allow better optimization of loops.

With mem barriers loops can't be optimized too much as code in compiler can not traverse loop back control through mem barriers to determine exit condition.

**Looking for feedback about possible impact.**

Partially discussed in, and extracted from https://mail.openjdk.java.net/pipermail/panama-dev/2021-June/014204.html

After

Benchmark                                       (size)  Mode  Cnt      Score      Error  Units
VectorCopySegments.copyWithNative                 1024  avgt   10     20.293 ?    0.436  ns/op
VectorCopySegments.copyWithNative              1048576  avgt   10  22270.840 ?  579.533  ns/op
VectorCopySegments.copyWithNativeShared           1024  avgt   10     15.854 ?    0.061  ns/op
VectorCopySegments.copyWithNativeShared        1048576  avgt   10  21948.236 ?   43.981  ns/op
VectorCopySegments.copyWithNativeToArray          1024  avgt   10     20.318 ?    0.347  ns/op
VectorCopySegments.copyWithNativeToArray       1048576  avgt   10  22142.499 ?  305.501  ns/op
VectorCopySegments.copyWithVector                 1024  avgt   10     31.240 ?    0.333  ns/op
VectorCopySegments.copyWithVector              1048576  avgt   10  25320.898 ?  118.397  ns/op
VectorCopySegments.copyWithVectorDirectBuffer     1024  avgt   10     21.605 ?    0.210  ns/op
VectorCopySegments.copyWithVectorDirectBuffer  1048576  avgt   10  23613.272 ? 1030.153  ns/op
VectorCopySegments.copyWithVectorShared           1024  avgt   10     19.897 ?    0.485  ns/op
VectorCopySegments.copyWithVectorShared        1048576  avgt   10  24719.767 ?  453.725  ns/op
VectorCopySegments.copyWithVectorShuffle          1024  avgt   10     36.364 ?    0.669  ns/op
VectorCopySegments.copyWithVectorShuffle       1048576  avgt   10  29730.528 ?  339.100  ns/op
VectorCopySegments.copyWithVectorToArray          1024  avgt   10     29.282 ?    0.338  ns/op
VectorCopySegments.copyWithVectorToArray       1048576  avgt   10  28502.004 ?  593.347  ns/op
VectorCopySegments.copyWithVectorUnroller         1024  avgt   10     36.368 ?    0.092  ns/op
VectorCopySegments.copyWithVectorUnroller      1048576  avgt   10  21528.433 ?  303.141  ns/op


Before

Benchmark                                       (size)  Mode  Cnt      Score      Error  Units
VectorCopySegments.copyWithVector                 1024  avgt   10     30.841 ?    0.119  ns/op
VectorCopySegments.copyWithVector              1048576  avgt   10  44834.639 ?  746.965  ns/op
VectorCopySegments.copyWithVectorShuffle          1024  avgt   10     47.797 ?    0.672  ns/op
VectorCopySegments.copyWithVectorShuffle       1048576  avgt   10  61171.416 ? 5656.479  ns/op
VectorCopySegments.copyWithVectorUnroller         1024  avgt   10     49.522 ?    1.027  ns/op
VectorCopySegments.copyWithVectorUnroller      1048576  avgt   10  72145.653 ?  987.111  ns/op

-------------

Commit messages:
 - Remove mem barriers after vector ops to increase performance

Changes: https://git.openjdk.java.net/panama-foreign/pull/566/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=566&range=00
  Stats: 10 lines in 1 file changed: 0 ins; 10 del; 0 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/566.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/566/head:pull/566

PR: https://git.openjdk.java.net/panama-foreign/pull/566