RFR: 8365200: RISC-V: compiler/loopopts/superword/TestGeneralizedReductions.java fails with Zvbb and vlen=128

[Dingli Zhang]

Hi all,
Please take a look and review this PR, thanks!

[JDK-8352529](https://bugs.openjdk.org/browse/JDK-8352529) enables this IR verification test for riscv. This test pass with zvbb when vlen=256, but fail when vlen=128.

The reason for the error is the same as [JDK-8357694](https://bugs.openjdk.org/browse/JDK-8357694). 2-element reductions for INT/LONG are not profitable, so the compiler won't generate the corresponding reductions IR.

This issue was not addressed together with [JDK-8357694](https://bugs.openjdk.org/browse/JDK-8357694) because the testMapReductionOnGlobalAccumulator use case where the error is reported has a different applyif method from other use cases: zvbb needs to be enabled.

### Test (fastdebug)
- [x] Run compiler/loopopts/superword/TestGeneralizedReductions.java on qemu-system w/ and w/o zvbb when vlen=256
- [x] Run compiler/loopopts/superword/TestGeneralizedReductions.java on qemu-system w/ and w/o zvbb when vlen=128

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

Commit messages:
 - 8365200: RISC-V: compiler/loopopts/superword/TestGeneralizedReductions.java fails with Zvbb and vlen=128

Changes: https://git.openjdk.org/jdk/pull/26719/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26719&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365200
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/26719.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26719/head:pull/26719

PR: https://git.openjdk.org/jdk/pull/26719