[jdk17u-dev] RFR: 8296545: C2 Blackholes should allow load optimizations [v2]

[Aleksey Shipilev]

> Unclean backport to improve Blackhole performance and match it to released JDK 20.
> The backport is unclean because there are two little conflicts:
>   1. The context conflict in `MemBarNode::make`, where the `MemBarStoreStore` was moved to another place. I resolved the conflict by hand: accepted only the `Blackhole` case removal.
>   2. IR Framework needs `MUL_I` constant for test to work.
> 
> Additional testing:
>   - [x] macos-aarch64-server-fastdebug, `compiler/irTests/blackhole` (includes new test)
>   - [x] Eyeballing perfasm outputs with/without this patch

Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge branch 'master' into JDK-8296545-blackhole-loads
 - Backport eab0ada3a16a432fdfd1f0b8fceca149c725451b

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

Changes:
  - all: https://git.openjdk.org/jdk17u-dev/pull/1273/files
  - new: https://git.openjdk.org/jdk17u-dev/pull/1273/files/1ab98c8f..eb33e255

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=1273&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=1273&range=00-01

  Stats: 2461 lines in 323 files changed: 1988 ins; 210 del; 263 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/1273.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/1273/head:pull/1273

PR: https://git.openjdk.org/jdk17u-dev/pull/1273