RFR: 8372285: G1: Micro-optimize x86 barrier code [v2]

[Aleksey Shipilev]

> We know from [JDK-8372284](https://bugs.openjdk.org/browse/JDK-8372284) that G1 C2 stubs can take ~10% of total instructions. So minor optimizations in hand-written assembly pay off for code density. This PR does a little x86-specific polishing: `testptr` where possible, short forward branches where possible. I rewired some code to make it abundantly clear the branches in question are short. It also makes clear that lots of the affected methods are essentially fall-through.
> 
> The patch is deliberately on simpler side, so we can backport it to 25u, if need arises.
> 
> Additional testing:
>  - [x] Linux x86_64 server fastdebug, `tier1`
>  - [ ]  Linux x86_64 server fastdebug, `all`

Aleksey Shipilev has updated the pull request incrementally with two additional commits since the last revision:

 - Comment
 - Shorten a few more branches

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/28446/files
  - new: https://git.openjdk.org/jdk/pull/28446/files/6250ea8a..edbc74a2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28446&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28446&range=00-01

  Stats: 4 lines in 1 file changed: 1 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/28446.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28446/head:pull/28446

PR: https://git.openjdk.org/jdk/pull/28446