RFR: 8378793: Add ResolvedFieldEntry is_valid assert

[David Holmes]

On Thu, 26 Feb 2026 20:32:02 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

> Please review this small change to add an assert for values in ResolvedFieldEntry, which was useful for debugging at one time.
> Tested with tier1-3

Is the entry immutable after `fill_in` has been called? Just wondering if that is the only place we need to do the checks - which seem pretty heavyweight. If something is amiss this doesn't seem to help track down where the bad value arose - so I would have expected validity checks when it is created and then when it is mutated, not when it is used.

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PR Review: https://git.openjdk.org/jdk/pull/29943#pullrequestreview-3863921896