RFR: 8312121: Fix -Wconversion warnings in tribool.hpp

[Coleen Phillimore]

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

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Commit messages:
 - 8312121: Fix -Wconversion warnings in tribool.hpp

Changes: https://git.openjdk.org/jdk/pull/14892/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14892&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312121
  Stats: 13 lines in 5 files changed: 7 ins; 1 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/14892.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14892/head:pull/14892

PR: https://git.openjdk.org/jdk/pull/14892