RFR: 8374545: Drop logic for unchecked nullness conversions

[Vicente Romero]

On Mon, 5 Jan 2026 17:48:00 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> This PR removes the recursive checks for nullness conversions.
> We might need more warnings in the future, but of a much simpler kind, so the best way forward is to restore the code in `Types` and `Check` closer to what's in mainline, and take it from there.
> 
> I've left two changes:
> * the sutyping test from `null` to `Foo!` still fails
> * a cast from `null` to `Foo!` still fails
> 
> We might want to detune these as warnings (or not).

lgtm

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Marked as reviewed by vromero (Committer).

PR Review: https://git.openjdk.org/valhalla/pull/1857#pullrequestreview-3627916923