RFR: 8300693: Lower the compile threshold and reduce the iterations of warmup loop in VarHandles tests

[Mandy Chung]

`java/lang/invoke/VarHandles` tests run with C1, C2 and tiered compilations and the test cases are executed in the warm up loop with 20000 iterations to verify C1, C2 intrinsics.  Default Tier4CompileThreshold is 15000.

This PR proposes to scale the compile threshold to 0.1 such that the warm up loop can be reduced to 2000 iterations.  This will speed up the test execution time.


Before:
make test-only TEST=open/test/jdk/java/lang/invoke/VarHandles  341.06s user 14.65s system 563% cpu 1:03.14 total

After:
make test-only TEST=open/test/jdk/java/lang/invoke/VarHandles  234.38s user 13.08s system 535% cpu 46.218 total

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Commit messages:
 - 8300693: Lower the compile threshold and reduce the iterations of warmup loop in VarHandles tests

Changes: https://git.openjdk.org/jdk/pull/12104/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12104&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300693
  Stats: 182 lines in 27 files changed: 54 ins; 30 del; 98 mod
  Patch: https://git.openjdk.org/jdk/pull/12104.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12104/head:pull/12104

PR: https://git.openjdk.org/jdk/pull/12104