RFR: 8327380: Add tests for Shenandoah barrier expansion optimization [v2]

[Kangcheng Xu]

> The Ideal graph for Shenandoah barrier expansion is optimized so that unnecessary checks are eliminated; however, currently there is no test cases to determine these optimizations are in effect.
> 
> Adding unit tests with the IR test framework will support related code changes in the future.

Kangcheng Xu 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 ten additional commits since the last revision:

 - Merge branch 'master' into test-shenandoah-barrier-expansion
 - update asserted IR phase
 - change assertion phase from MACRO_EXPANSION to AFTER_MACRO_EXPANSION_STEP
 - add license header
 - update test annotation
 - Merge branch 'master' into test-shenandoah-barrier-expansion
 - update vm flags
 - update vm flags
 - add TestShenandoahBarrierExpansion

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18814/files
  - new: https://git.openjdk.org/jdk/pull/18814/files/25801ce6..f20f760c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18814&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18814&range=00-01

  Stats: 238005 lines in 4773 files changed: 141983 ins; 72944 del; 23078 mod
  Patch: https://git.openjdk.org/jdk/pull/18814.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18814/head:pull/18814

PR: https://git.openjdk.org/jdk/pull/18814