RFR: 8312121: Fix -Wconversion warnings in tribool.hpp [v6]

[David Holmes]

On Thu, 27 Jul 2023 22:50:16 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> Assigning _value first, and then doing _value | 2 doesn't get -Wconversion warnings.  Also, reduced include file inclusion a little.
>> Tested with tier1 on linux-x64-debug, windows-x64-debug, macos-aarch64-debug
>
> Coleen Phillimore has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - David suggestion
>  - David suggestion

Marked as reviewed by dholmes (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/14892#pullrequestreview-1550988170