RFR: 8294865: x86: Improve the code generation of MulVB and MulVL

[Quan Anh Mai]

Hi,

This patch simplifies and improves the code generation of `MulVB` and `MulVL` nodes,

- MulVB can be implemented by alternating `vmullw` on odd and even-index elements and combining the results.
- MulVL can be implemented on non-avx512dq by computing the product of each 32-bit half and adding the results together.

Vector API benchmark shows the results of `MUL` operations:

                                                  Before                After
    Benchmark          (size)   Mode  Cnt      Score     Error      Score     Error   Units   Change
    Byte64Vector.MUL     1024  thrpt   15   8948.607 ± 194.646   8860.404 ± 203.109  ops/ms   -0.99%
    Byte128Vector.MUL    1024  thrpt   15  12915.839 ± 291.262  13554.662 ± 488.695  ops/ms   +4.95%
    Byte256Vector.MUL    1024  thrpt   15  12129.959 ± 245.710  23279.276 ± 669.725  ops/ms  +91.92%
    Long128Vector.MUL    1024  thrpt   15   1183.663 ±  36.440   1489.892 ±  35.356  ops/ms  +25.87%
    Long256Vector.MUL    1024  thrpt   15   1911.802 ±  95.304   2834.088 ±  77.647  ops/ms  +48.24%

Please have a look and have some reviews, thank you very much.

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

Commit messages:
 - add vmulB for 8 bytes
 - Merge branch 'master' into improveMulVB
 - Merge branch 'master' into improveMulVB
 - Merge branch 'master' into improveMulVB
 - fix
 - mulV

Changes: https://git.openjdk.org/jdk/pull/10571/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10571&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294865
  Stats: 161 lines in 5 files changed: 11 ins; 62 del; 88 mod
  Patch: https://git.openjdk.org/jdk/pull/10571.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10571/head:pull/10571

PR: https://git.openjdk.org/jdk/pull/10571