RFR: 8320921: GHA: Parallelize hotspot_compiler test jobs

[Aleksey Shipilev]

In current GHA, `hotspot_compiler` testing takes a long time, and often takes the longest. On MacOS and Windows it routinely takes 60..80 minutes, while other test groups run in 30..40 minutes. This often drags the total wall clock time to run the GHA up. Fortunately, there are already several parts in `hotspot_compiler` test group, which we might just run separately:
https://github.com/openjdk/jdk/blob/dc256fbc6490f8163adb286dbb7380c10e5e1e06/test/hotspot/jtreg/TEST.groups#L142-L146

There is a bit of awkwardness about `not_xcomp` subgroup, which does not have lots of tests. Instead of doing another short job, I think we can just meld it into the quickest "part 2".


# Before
linux-x64 / test (hs/tier1 compiler) 38m 27s 	
windows-x64 / test (hs/tier1 compiler): 1h 17m 30s 	
macos-x64 / test (hs/tier1 compiler): 1h 2m 53s 

# After
linux-x64 / test (hs/tier1 compiler part 1): 11m 38s 	
linux-x64 / test (hs/tier1 compiler part 2): 9m 34s 	
linux-x64 / test (hs/tier1 compiler part 3): 21m 40s 	

windows-x64 / test (hs/tier1 compiler part 1): 24m 19s 	
windows-x64 / test (hs/tier1 compiler part 2): 18m 54s 	
windows-x64 / test (hs/tier1 compiler part 3): 43m 32s 	

macos-x64 / test (hs/tier1 compiler part 1): 26m 44s 	
macos-x64 / test (hs/tier1 compiler part 2): 20m 40s	
macos-x64 / test (hs/tier1 compiler part 3): 42m 10s


Additional testing:
 - [x] GHA (current run is in progress, but I had clean runs before)

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

Commit messages:
 - Job name is actually wrong
 - Move xcomp to part 2
 - Work

Changes: https://git.openjdk.org/jdk/pull/16860/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16860&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320921
  Stats: 13 lines in 1 file changed: 10 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/16860.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16860/head:pull/16860

PR: https://git.openjdk.org/jdk/pull/16860