RFR: 8257146: C2: extend the scope of StressGCM

[Roberto Castañeda Lozano]

Extend the scope of StressGCM by allowing it to move instructions into basic blocks with worse frequency. This should improve StressGCM's ability to expose bugs where C2 relies on GCM heuristics for correctness.

Tested with StressGCM enabled on `hs-tier1-3` and a single repetition, the following test cases fail on `linux-aarch64-debug` and require further investigation:
- `serviceability/sa/ClhsdbJstackXcompStress.java`
- `compiler/c2/cr6663848/Tester.java`

Also tested with StressGCM disabled (default)  on `hs-tier1-3`, all tests pass.

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

Commit messages:
 - 8257146: C2: extend the scope of StressGCM

Changes: https://git.openjdk.java.net/jdk/pull/1469/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1469&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257146
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1469.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1469/head:pull/1469

PR: https://git.openjdk.java.net/jdk/pull/1469