RFR: 8261914: IfNode::fold_compares_helper faces non-canonicalized bool when running JRuby JSON workload

[Roland Westrelin]

The assert fires because the current IfNode's condition was not
canonicalized when it's being folded with a dominating
IfNode. idealize_test() should have taken care of that because it
executed before fold_compares() but it didn't because:

  Node* new_b = phase->transform( new BoolNode(b->in(1), bt.negate()) );
  if( !new_b->is_Bool() ) return NULL;

caused it to bail out. new_b is a constant. This happens because of
the order in which nodes are processed by IGVN. The If's current Bool
would also constant fold but it's in the IGVN worklist and hasn't been
processed yet.

The fix I propose is to keep Aleksey's defensive fix but to check that
the Bool input is indeed about to be transformed by IGVN and that that
would cause the IfNode to be reprocessed.

I tried to write a test case but didn't succeed. The 2 If nodes come
from a tableswitch that's transformed into a series of If based on
profile data. I couldn't reproduce the profile data with a simple test
case.

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Commit messages:
 - fix

Changes: https://git.openjdk.java.net/jdk/pull/2707/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2707&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261914
  Stats: 9 lines in 1 file changed: 3 ins; 4 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2707.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2707/head:pull/2707

PR: https://git.openjdk.java.net/jdk/pull/2707