RFR: 8341293: Split field loads through Nested Phis [v9]

[Dhamoder Nalla]

> This enhances the changes introduced in [JDK PR 12897](https://github.com/openjdk/jdk/pull/12897) by handling nested Phi nodes (phi -> phi -> AddP -> Load*) during scalar replacement. The primary goal is to split field loads (AddP -> Load*) involving nested Phi parent nodes, thereby increasing opportunities for scalar replacement and reducing memory allocations.
> 
> 
> **Here is an illustration of the sequence of Ideal Graph Transformations applied to split through nested `Phi` nodes.**
>  
> **1. Initial State (Before Transformation)**
> The graph contains a nested Phi structure where two Allocate nodes merge via a Phi node.
> 
> ![image](https://github.com/user-attachments/assets/c18e5ca0-c554-475c-814a-7cb288d96569)
> 
> **2. After Splitting Through Child Phi**
> The transformation separates field loads by introducing additional AddP and Load nodes for each Allocate input.
> 
> ![image](https://github.com/user-attachments/assets/b279b5f2-9ec6-4d9b-a627-506451f1cf81)
> 
> **3. After Splitting Load Field Through Parent Phi**
> The field load operation (Load) is pushed even further up in the graph.
> 
> Instead of merging AddP pointers in a Phi node and then performing a Load, the transformation ensures that each path has its AddP -> Load sequence before merging.
> 
> This further eliminates the need to perform field loads on a Phi node, making the graph more conducive to scalar replacement.
> 
> ![image](https://github.com/user-attachments/assets/f506b918-2dd0-4dbe-a440-ff253afa3961)
> 
> ### JMH Benchmark Results:
> 
> #### With Disabled RAM
> 
> | Benchmark | Mode | Count | Score | Error | Units |
> |-----------|------|-------|-------|-------|-------|
> | testBailOut_runner | avgt | 15 | 13.969 | ± 0.248 | ms/op |
> | testFieldEscapeWithMerge_runner | avgt | 15 | 80.300 | ± 4.306 | ms/op |
> | testMerge_TryCatchFinally_runner | avgt | 15 | 72.182 | ± 1.781 | ms/op |
> | testMultiParentPhi_runner | avgt | 15 | 2.983 | ± 0.001 | ms/op |
> | testNestedPhiPolymorphic_runner | avgt | 15 | 18.342 | ± 0.731 | ms/op |
> | testNestedPhiProcessOrder_runner | avgt | 15 | 14.315 | ± 0.443 | ms/op |
> | testNestedPhiWithLambda_runner | avgt | 15 | 18.511 | ± 1.212 | ms/op |
> | testNestedPhiWithTrap_runner | avgt | 15 | 66.277 | ± 1.478 | ms/op |
> | testNestedPhi_FieldLoad_runner | avgt | 15 | 17.968 | ± 0.306 | ms/op |
> | testNestedPhi_TryCatch_runner | avgt | 15 | 14.186 | ± 0.247 | ms/op |
> | testRematerialize_MultiObj_runner | avgt | 15 | 88.435 | ± 4.869 | ms/op |
> | testRematerialize_SingleObj_runner | avgt | 15 | 29560.130 | ± 48.797 ...

Dhamoder Nalla has updated the pull request incrementally with one additional commit since the last revision:

  address CR comments

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/21270/files
  - new: https://git.openjdk.org/jdk/pull/21270/files/3c56f98d..7947053b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21270&range=08
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21270&range=07-08

  Stats: 8 lines in 3 files changed: 0 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/21270.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21270/head:pull/21270

PR: https://git.openjdk.org/jdk/pull/21270