[lworld] RFR: Regression~30% in jvm2008 Serial with -XX:-EnableValhalla

[Chen Liang]

There's a spike of calls to `Unsafe.isFlatField` in serialzation workloads. The layout information can be prepared with FieldReflector and cached. CI looks good, need to verify the regression is fixed.

The old perfasm profile was:
....[Hottest Regions]...............................................................................
   4.66% libjvm.so Unsafe_IsFlatField
   4.13% c2, level 4 java.io.ObjectOutputStream::writeObject0, version 6, compile id 2698
   3.72% c2, level 4 java.io.ObjectInputStream$BlockDataInputStream::readUTFSpan, version 2, compile id 1878
   3.61% c2, level 4 java.io.ObjectInputStream$PeekInputStream::readFully, version 2, compile id 1902

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Commit messages:
 - Bug
 - 8369719: [lworld] Regression~30% in jvm2008 Serial with -XX:-EnableValhalla

Changes: https://git.openjdk.org/valhalla/pull/1676/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1676&range=00
  Stats: 9 lines in 1 file changed: 4 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/valhalla/pull/1676.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1676/head:pull/1676

PR: https://git.openjdk.org/valhalla/pull/1676