RFR: 8266256: compiler.vectorization.TestBufferVectorization does testing twice

[Igor Ignatyev]

Hi all,

could you please review this patch that removes work duplication within `TestBufferVectorization` test? 

from JBS:
> `compiler.vectorization.TestBufferVectorization` runs the testing twice, one time in the JVM created by jtreg, and once again by the JVM created by TestBufferVectorization.

after the duplication was removed, it became possible to change the execution mode to `driver`, which is done as part of this patch as well as small refactoring.

Thanks,
-- Igor

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Commit messages:
 - use j.t.lib.Platform::is32bit
 - use switch expression
 - switch to driver mode
 - remove work duplication

Changes: https://git.openjdk.java.net/jdk/pull/3781/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3781&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266256
  Stats: 80 lines in 1 file changed: 30 ins; 42 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3781.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3781/head:pull/3781

PR: https://git.openjdk.java.net/jdk/pull/3781