RFR: 8341708: Optimize safepoint poll encoding with smaller poll data offset [v2]

[Aleksey Shipilev]

> See the bug for discussion. We can optimize the encoding for safepoint polls by making sure the polling data is at small offset in `Thread`. There is already the area where we pull data like this for better encoding.
> 
> Code density improves on x86_64, can be seen with just `-Xcomp -XX:+CITime`:
> 
> 
> # Before
>   nmethod code size         :  7107136 bytes
>   nmethod code size         :  7107120 bytes
>   nmethod code size         :  7107136 bytes
> 
> # After (-0.25%)
>   nmethod code size         :  7088896 bytes
>   nmethod code size         :  7088896 bytes
>   nmethod code size         :  7088864 bytes
> 
> 
> Additional testing:
>  - [x] Ad-hoc code density checks
>  - [x] Linux x86_64 server fastdebug, `all`

Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:

  Shuffle things around: keep the accessors in JavaThread

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/21405/files
  - new: https://git.openjdk.org/jdk/pull/21405/files/667c227d..5ca02006

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21405&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21405&range=00-01

  Stats: 42 lines in 2 files changed: 21 ins; 20 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21405.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21405/head:pull/21405

PR: https://git.openjdk.org/jdk/pull/21405