RFR: JDK-8287061: Support for rematerializing scalar replaced objects participating in allocation merges [v12]

[Cesar Soares Lucas]

> Can I please get reviews for this PR? 
> 
> The most common and frequent use of NonEscaping Phis merging object allocations is for debugging information. The two graphs below show numbers for Renaissance and DaCapo benchmarks - similar results are obtained for all other applications that I tested.
> 
> With what frequency does each IR node type occurs as an allocation merge user? I.e., if the same node type uses a Phi N times the counter is incremented by N:
> 
> ![image](https://user-images.githubusercontent.com/2249648/222280517-4dcf5871-2564-4207-b49e-22aee47fa49d.png)
> 
> What are the most common users of allocation merges? I.e., if the same node type uses a Phi N times the counter is incremented by 1:
> 
> ![image](https://user-images.githubusercontent.com/2249648/222280608-ca742a4e-1622-4e69-a778-e4db6805ea02.png)
> 
> This PR adds support scalar replacing allocations participating in merges used as debug information OR as a base for field loads. I plan to create subsequent PRs to enable scalar replacement of merges used by other node types (CmpP is next on the list) subsequently.
> 
> The approach I used for _rematerialization_ is pretty straightforward. It consists basically of the following. 1) New IR node (suggested by V. Kozlov), named SafePointScalarMergeNode, to represent a set of SafePointScalarObjectNode; 2) Each scalar replaceable input participating in a merge will get a SafePointScalarObjectNode like if it weren't part of a merge. 3) Add a new Class to support the rematerialization of SR objects that are part of a merge; 4) Patch HotSpot to be able to serialize and deserialize debug information related to allocation merges; 5) Patch C2 to generate unique types for SR objects participating in some allocation merges.
> 
> The approach I used for _enabling the scalar replacement of some of the inputs of the allocation merge_ is also pretty straightforward: call `MemNode::split_through_phi` to, well, split AddP->Load* through the merge which will render the Phi useless.
> 
> I tested this with JTREG tests tier 1-4 (Windows, Linux, and Mac) and didn't see regression. I also experimented with several applications and didn't see any failure. I also ran tests with "-ea -esa -Xbatch -Xcomp -XX:+UnlockExperimentalVMOptions -XX:-TieredCompilation -server -XX:+IgnoreUnrecognizedVMOptions -XX:+UnlockDiagnosticVMOptions -XX:+StressLCM -XX:+StressGCM -XX:+StressCCP" and didn't observe any related failures.

Cesar Soares Lucas has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 12 commits:

 - Merge remote-tracking branch 'origin/master' into rematerialization-of-merges
 - Address part of PR review 4 & fix a bug setting only_candidate
 - Catching up with master
   
   Merge remote-tracking branch 'origin/master' into rematerialization-of-merges
 - Fix tests. Remember previous reducible Phis.
 - Address PR review 3. Some comments and be able to abort compilation.
 - Merge with Master
 - Addressing PR review 2: refactor & reuse MacroExpand::scalar_replacement method.
 - Address PR feeedback 1: make ObjectMergeValue subclass of ObjectValue & create new IR class to represent scalarized merges.
 - Add support for SR'ing some inputs of merges used for field loads
 - Fix some typos and do some small refactorings.
 - ... and 2 more: https://git.openjdk.org/jdk/compare/561ec9c5...542c5ef1

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

Changes: https://git.openjdk.org/jdk/pull/12897/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12897&range=11
  Stats: 2250 lines in 26 files changed: 1990 ins; 108 del; 152 mod
  Patch: https://git.openjdk.org/jdk/pull/12897.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/12897/head:pull/12897

PR: https://git.openjdk.org/jdk/pull/12897