[lworld] RFR: 8375441: [lworld] C2: assert(is_instance()) failed: bad cast [v4]

[Marc Chevalier]

> Some code added by [JDK-8372700](https://bugs.openjdk.org/browse/JDK-8372700) can compute the constant value of a field of a (flatten) element in a flat array. We get a crash when the element of the array is known to be `null`, and so the field doesn't exist.
> 
> So, let's just check in `ciConstant ciFlatArray::field_value(int index, ciField* field)` whether we get a null constant before interpreting it as a `ciInstance` and trying to retrieve a field from there. This should be enough since a `ciObject` is (directly) derived by `ciNullObject`, `ciInstance` and `ciArray`. Since we are looking up a value of a flat array, an element cannot be a `ciArray` (arrays have identities and can't be contained in a flat array). After looking up whether the flat array element is null, the `obj->as_instance()` cast acts as an assert, should we ever add another derived class from `ciObject`.
> 
> In case of a null array element, `field_value` simply returns an invalid `ciConstant`.
> 
> Tested with tier1,tier2,tier3,hs-precheckin-comp,hs-comp-stress,valhalla-comp-stress. Looks good.
> 
> Thanks,
> Marc

Marc Chevalier has updated the pull request incrementally with one additional commit since the last revision:

  More

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Changes:
  - all: https://git.openjdk.org/valhalla/pull/1923/files
  - new: https://git.openjdk.org/valhalla/pull/1923/files/34be1bd4..f029b762

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1923&range=03
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1923&range=02-03

  Stats: 13 lines in 2 files changed: 0 ins; 0 del; 13 mod
  Patch: https://git.openjdk.org/valhalla/pull/1923.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1923/head:pull/1923

PR: https://git.openjdk.org/valhalla/pull/1923