[lworld] RFR: 8375441: [lworld] C2: assert(is_instance()) failed: bad cast [v5]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 19 additional commits since the last revision:

 - Merge remote-tracking branch 'origin/lworld' into JDK-8375441.review
 - Merge remote-tracking branch 'origin/lworld' into JDK-8375441.review
 - Unproblem-list
 - Merge remote-tracking branch 'origin/lworld' into JDK-8375441.review
 - More
 - Addressing more comments
 - Merge
 - Some reviews
 - Details
 - exception handling
 - ... and 9 more: https://git.openjdk.org/valhalla/compare/91aea67f...90f8677d

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

Changes:
  - all: https://git.openjdk.org/valhalla/pull/1923/files
  - new: https://git.openjdk.org/valhalla/pull/1923/files/f029b762..90f8677d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1923&range=04
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1923&range=03-04

  Stats: 4128 lines in 133 files changed: 1088 ins; 2546 del; 494 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