RFR: 8354853: Clean up x86 registers after 32-bit x86 removal

[Andrew Haley]

On Wed, 14 Jan 2026 09:37:09 GMT, Manuel Hässig <mhaessig at openjdk.org> wrote:

> This PR cleans up some 32-bit remnants in the x86 register code. This also presented the opportunity to convert the unscoped enums into typed constants.
> 
> Testing:
>  - [ ] Github Actions
>  - [ ] tier1,tier2 on linux-x64-debug, linux-x64, windows-x64-debug, windows-x64, macosx-x64-debug, macosx-x64

Looks good.

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Marked as reviewed by aph (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/29224#pullrequestreview-3660145106