[jdk21u-dev] RFR: 8309685: Fix -Wconversion warnings in assembler and register code

[Aleksey Shipilev]

The backport is not clean because [JDK-8331421](https://bugs.openjdk.org/browse/JDK-8331421) was backported ahead of it.

Additional testing:
 - [ ] Linux x86_64 server fastdebug, `all` tests pass

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Commit messages:
 - Backport 230bcb769a2701b80b4a12a39785f88a1db48ba9

Changes: https://git.openjdk.org/jdk21u-dev/pull/567/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=567&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309685
  Stats: 60 lines in 20 files changed: 0 ins; 0 del; 60 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/567.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/567/head:pull/567

PR: https://git.openjdk.org/jdk21u-dev/pull/567