8309685: Fix -Wconversion warnings in assembler and register code

[shipilev]

---

Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue
• JDK-8309685 needs maintainer approval
• JDK-8331421 needs maintainer approval

Issues

• JDK-8309685: Fix -Wconversion warnings in assembler and register code (Enhancement - P4)
• JDK-8331421: ubsan: vmreg.cpp checking error member call on misaligned address (Bug - P4)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk21u-dev.git pull/567/head:pull/567
$ git checkout pull/567

Update a local copy of the PR:
$ git checkout pull/567
$ git pull https://git.openjdk.org/jdk21u-dev.git pull/567/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 567

View PR using the GUI difftool:
$ git pr show -t 567

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk21u-dev/pull/567.diff

[bridgekeeper]

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[openjdk]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[openjdk]

This backport pull request has now been updated with issue from the original commit.

[shipilev]

/issue add JDK-8331421

[openjdk]

@shipilev
Adding additional issue to issue list: 8331421: ubsan: vmreg.cpp checking error member call on misaligned address.