RFR: JDK-8319256: Print more diagnostic information when an unexpected user is found in a Phi [v3]

[Cesar Soares Lucas]

> Can I please get some review on this PR to:
> 
> - Add more verification around the implementation of reduce allocation merges.
> - Print more diagnostic information when an unexpected situation is encountered.
> 
> Testing: Linux x64 tier1-4, Renaissance, Dacapo and SpecJBB.

Cesar Soares Lucas has updated the pull request incrementally with two additional commits since the last revision:

 - Update src/hotspot/share/opto/c2_globals.hpp
   
   Co-authored-by: Tobias Hartmann <tobias.hartmann at oracle.com>
 - Update src/hotspot/share/opto/c2_globals.hpp
   
   Co-authored-by: Tobias Hartmann <tobias.hartmann at oracle.com>

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/16465/files
  - new: https://git.openjdk.org/jdk/pull/16465/files/70312fb7..474214be

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16465&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16465&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/16465.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16465/head:pull/16465

PR: https://git.openjdk.org/jdk/pull/16465