RFR: 8328998: Encoding support for Intel APX extended general-purpose registers [v5]

[Steve Dohrmann]

> Add instruction encoding support for Intel APX extended general-purpose registers:
> 
> Intel Advanced Performance Extensions (APX) doubles the number of general-purpose registers, from 16 to 32. For more information about APX, see https://www.intel.com/content/www/us/en/developer/articles/technical/advanced-performance-extensions-apx.html.
> 
> By specification, instruction encoding remains unchanged for instructions using only the lower 16 GPRs. For cases where one or more instruction operands reference extended GPRs (Egprs), encoding targets either REX2, an extension of REX encoding, or an extended version of EVEX encoding. These new encoding schemes extend or modify existing instruction prefixes only when Egprs are used.

Steve Dohrmann has updated the pull request incrementally with one additional commit since the last revision:

  from review comments: simplification, fix comments and white space

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Changes:
  - all: https://git.openjdk.org/jdk/pull/18476/files
  - new: https://git.openjdk.org/jdk/pull/18476/files/eb246fd7..7bd4b885

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18476&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18476&range=03-04

  Stats: 68 lines in 2 files changed: 0 ins; 2 del; 66 mod
  Patch: https://git.openjdk.org/jdk/pull/18476.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18476/head:pull/18476

PR: https://git.openjdk.org/jdk/pull/18476