[lworld] RFR: 8357474: [lworld] Consolidate load/store flat inline types

[Quan Anh Mai]

Hi,

This patch consolidates loading and storing of flat inline types. This makes it so that `make_from_flat` and `store_flat` will take a pointer to the target location. `make_from_flat_array` and `store_flat_array` are split out into their own methods which receive an `idx` parameter. This also fixes some issues I theorycrafted:

- Improper loading/storing of atomic elements in a non-atomic container. This is not supported at the moment but it would be great to prepare for it.
- `is_naturally_atomic` checks `nof_declared_nonstatic_fields`, which is incorrect if the sole field has multiple fields. I refactored it into `ciInlineKlass`, the decision to do atomic loads/stores is also delegated to the callee, the caller only needs to know whether the operations act as if they are atomic, not whether they are actually executed atomically.
- `null_free` of an oop can only be trusted if the container itself is null-free. For example this case:

        value class StringHolder {
            @NullRestricted
            String s;
        }

        value class StringHolderHolder {
            // flatten
            StringHolder v;
        }

Then `v.s` cannot be trusted to be non-null because `v` can be null.

This also allows straightforward implementation of https://bugs.openjdk.org/browse/JDK-8349110.

Please let me know what you think. Thanks very much.

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

Commit messages:
 - add tests
 - Consolidate load/store of flat values

Changes: https://git.openjdk.org/valhalla/pull/1470/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1470&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357474
  Stats: 743 lines in 10 files changed: 330 ins; 261 del; 152 mod
  Patch: https://git.openjdk.org/valhalla/pull/1470.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1470/head:pull/1470

PR: https://git.openjdk.org/valhalla/pull/1470