RFR: 8351994: Enable Extended EVEX to REX2/REX demotion when src and dst are the same [v27]

[Srinivas Vamsi Parasa]

> Intel APX NDD instructions are encoded using EVEX encoding. The goal of this PR is to enable optimized instruction encoding for Intel APX NDD instructions when the non-destructive destination is same as the first source.
> 
> For example:
> 
> `eaddl r18, r18, r25` can be encoded as `addl r18, r25` using APX REX2 encoding
> `eaddl r2, r2, r7` can be encoded as `addl r2, r7` using non-APX legacy encoding

Srinivas Vamsi Parasa has updated the pull request incrementally with one additional commit since the last revision:

  Update x86-asmtest.py to enable demotion by default and make test generation optional

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Changes:
  - all: https://git.openjdk.org/jdk/pull/24431/files
  - new: https://git.openjdk.org/jdk/pull/24431/files/65656aae..b2e8fd2a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24431&range=26
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24431&range=25-26

  Stats: 2757 lines in 2 files changed: 204 ins; 356 del; 2197 mod
  Patch: https://git.openjdk.org/jdk/pull/24431.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24431/head:pull/24431

PR: https://git.openjdk.org/jdk/pull/24431