RFR: JDK-8287061: Support for rematerializing scalar replaced objects participating in allocation merges [v15]

[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 incrementally with one additional commit since the last revision:

  Address PR review 6: debug format output & some refactoring.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12897/files
  - new: https://git.openjdk.org/jdk/pull/12897/files/8f81a7c8..3a5ed401

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12897&range=14
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12897&range=13-14

  Stats: 112 lines in 6 files changed: 37 ins; 59 del; 16 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