RFR: 8291729: Reduce runtime of vm.compiler microbenchmarks

[Eric Caspole]

The current run time is about 1 day 6 hours. With these changes the default run time is reduced to about 5h45m with results consistent enough for weekly testing based on internal testing.

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

Commit messages:
 - Merge
 - 8291729: Reduce runtime of vm.compiler microbenchmarks

Changes: https://git.openjdk.org/jdk/pull/9728/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9728&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291729
  Stats: 198 lines in 42 files changed: 168 ins; 2 del; 28 mod
  Patch: https://git.openjdk.org/jdk/pull/9728.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9728/head:pull/9728

PR: https://git.openjdk.org/jdk/pull/9728