RFR: 8343941: IGV: dump graph at different register allocation steps [v2]

[Roberto Castañeda Lozano]

> This changeset dumps C2's low-level intermediate representation at the following intermediate register allocation points:
> - Initial liveness: after initial liveness information is computed.
> - Aggressive coalescing: after aggressively coalescing live ranges and destructing SSA.
> - Initial spilling: after initial round of spilling derived from physical interference graph construction.
> - Conservative coalescing: after each round of conservative (colorability-preserving) coalescing (if `OptoCoalesce` is enabled).
> - Iterative spilling: after each round of spilling.
> - After iterative spilling: after the main register allocation loop.
> - Post-allocation copy removal: after peephole copy removal.
> - Merge multiple definitions: after local merging of equivalent nodes related by the same live range.
> - Fix up spills: convert load-store spills into memory operand accesses ("CISC spilling") if allowed by the target platform and `UseCISCSpill` is enabled.
> 
> The new dumps have already proved to be useful in the investigation of [JDK-8331295](https://bugs.openjdk.org/browse/JDK-8331295).
> 
> #### Testing
> 
> - tier1-3 (windows-x64, linux-x64, linux-aarch64, macosx-x64, and macosx-aarch64).
> - Tested automatically that dumping, scheduling, and viewing hundreds of the new graphs does not trigger any failure on HotSpot or IGV (by instrumenting IGV to schedule and view graphs eagerly and running `java -Xbatch -XX:PrintIdealGraphLevel=4`).

Roberto Castañeda Lozano has updated the pull request incrementally with one additional commit since the last revision:

  Split FIXUP_SPILLS

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/22017/files
  - new: https://git.openjdk.org/jdk/pull/22017/files/90f9a24e..e44fa796

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=22017&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=22017&range=00-01

  Stats: 3 lines in 3 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/22017.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22017/head:pull/22017

PR: https://git.openjdk.org/jdk/pull/22017