[lworld] RFR: [lworld] Field::get should return default value instead of null on non-flattened inline field

[Mandy Chung]

A field could be of a non-flattened inline type.  `Unsafe::getReference(Object o, long offset, Class<?> valueType)` should be used instead.   The value type parameter is used to differentiate if the field is an inline type or not; if so, it will return an uninitialized value of the inline type.

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Commit messages:
 - test fix
 - clean up
 - add new regression test
 - JDK-8260371: [lworld] Field::get should return default value instead of null on non-flattened inline field

Changes: https://git.openjdk.java.net/valhalla/pull/314/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=314&range=00
  Stats: 166 lines in 5 files changed: 159 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/314.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/314/head:pull/314

PR: https://git.openjdk.java.net/valhalla/pull/314