RFR: 8305073: Fix VerifyLoopOptimizations - step 2 - verify idom [v3]

[Emanuel Peter]

> This is the second step in the `VerifyLoopOptimizations` revival.
> 
> Last step:
> [JDK-8173709](https://bugs.openjdk.org/browse/JDK-8173709) Fix VerifyLoopOptimizations - step 1 - minimal infrastructure 
> See PR for all the planned steps: https://github.com/openjdk/jdk/pull/13207
> 
> Bug fixing for this step:
> [JDK-8308084](https://bugs.openjdk.org/browse/JDK-8308084) C2 fix idom bug in PhaseIdealLoop::create_new_if_for_predicate
> (https://github.com/openjdk/jdk/pull/13980)
> 
> Next step:
> [JDK-8307982](https://bugs.openjdk.org/browse/JDK-8307982) Fix VerifyLoopOptimizations - step 3 - fix ctrl/loop
> 
> I added `TestVerifyLoopOptimizations.java` per @robcasloz 's request. It works just like `TestVerifyIterativeGVN.java`, with a simple `-Xcomp -XX:+VerifyLoopOptimizations` on a basically empty test. It fails until this patch is integrated: https://github.com/openjdk/jdk/pull/13980

Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:

  add TestVerifyLoopOptimizations.java

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/13951/files
  - new: https://git.openjdk.org/jdk/pull/13951/files/56f66361..3963e133

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13951&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13951&range=01-02

  Stats: 37 lines in 1 file changed: 37 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/13951.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13951/head:pull/13951

PR: https://git.openjdk.org/jdk/pull/13951