RFR: 8289943: Simplify some object allocation merges

[Cesar Soares]

Hi there, can I please get some feedback on this approach to simplify object allocation merges in order to promote Scalar Replacement of the objects involved in the merge?

The basic idea for this [approach was discussed in this thread](https://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2022-April/055189.html) and it consists of: 
1) Identify Phi nodes that merge object allocations and replace them with a new IR node called ReducedAllocationMergeNode (RAM node).
2) Scalar Replace the incoming allocations to the RAM node.
3) Scalar Replace the RAM node itself.

There are a few conditions for doing the replacement of the Phi by a RAM node though - Although I plan to work on removing them in subsequent PRs:

- ~~The original Phi node should be merging Allocate nodes in all inputs.~~
- The only supported users of the original Phi are AddP->Load, SafePoints/Traps, DecodeN.

These are the critical parts of the implementation and I'd appreciate it very much if you could tell me if what I implemented isn't violating any C2 IR constraints:

- The way I identify/use the memory edges that will be used to find the last stored values to the merged object fields.
- The way I check if there is an incoming Allocate node to the original Phi node.
- The way I check if there is no store to the merged objects after they are merged.

Testing:
- Linux. fastdebug -> hotspot_all, renaissance, dacapo

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

Commit messages:
 - Lift requirement for all inputs to be Allocate.
 - fix formatting
 - merge fix
 - work
 - work
 - work
 - work
 - work
 - work
 - work
 - ... and 14 more: https://git.openjdk.org/jdk/compare/68c5957b...e1d506f3

Changes: https://git.openjdk.org/jdk/pull/9073/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9073&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289943
  Stats: 1718 lines in 21 files changed: 1670 ins; 6 del; 42 mod
  Patch: https://git.openjdk.org/jdk/pull/9073.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9073/head:pull/9073

PR: https://git.openjdk.org/jdk/pull/9073