RFR: 8313564: Fix -Wconversion warnings in classfile code [v4]

[Coleen Phillimore]

On Thu, 3 Aug 2023 01:37:08 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> This patch fixes various -Wconversion warnings in classfile code.  I broke the change into commits so the changes are easier to see.
>> Tested with tier1-4.
>
> Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix Atomic items_added code.

I reverted the items_count change.

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PR Review: https://git.openjdk.org/jdk/pull/15111#pullrequestreview-1560950292