RFR: 8308084: C2 fix idom bug in PhaseIdealLoop::create_new_if_for_predicate

[Emanuel Peter]

Bug fixing to make the verification pass for https://github.com/openjdk/jdk/pull/13951 [JDK-8305073](https://bugs.openjdk.org/browse/JDK-8305073).

There only seemed to be one bug with idom that I could find up to **tier6 and stress testing**. That one bug already showed up with a simple `java -Xcomp --version`. But it is possible that there are more that we would find in the future, maybe with the fuzzer.

**Details about the bug** I fixed in `PhaseIdealLoop::create_new_if_for_predicate`:
We computed the `dom_lca_internal` for `rgn` too early - the following line can change the CFG such that the idom would change:
https://github.com/openjdk/jdk/blob/1e1abc4c086298060ccb13b63f646a298bbe3ef7/src/hotspot/share/opto/loopPredicate.cpp#L216

So I moved the idom computation down, until after we do not change the CFG anymore, and idom should be stable from there on.

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Commit messages:
 - 8308084: C2 fix idom bug in PhaseIdealLoop::create_new_if_for_predicate

Changes: https://git.openjdk.org/jdk/pull/13980/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13980&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308084
  Stats: 15 lines in 1 file changed: 8 ins; 7 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/13980.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13980/head:pull/13980

PR: https://git.openjdk.org/jdk/pull/13980