RFR: 8294865: x86: Improve the code generation of MulVB and MulVL [v3]

[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.

Quan Anh Mai has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains eight commits:

 - Merge branch 'master' into improveMulVB
 - refactor conditions
 - add vmulB for 8 bytes
 - Merge branch 'master' into improveMulVB
 - Merge branch 'master' into improveMulVB
 - Merge branch 'master' into improveMulVB
 - fix
 - mulV

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Changes: https://git.openjdk.org/jdk/pull/10571/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10571&range=02
  Stats: 170 lines in 5 files changed: 15 ins; 64 del; 91 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