RFR(S): 8243670: Unexpected test result caused by C2 MergeMemNode::Ideal

[Roland Westrelin]

> I added some extra code to check if we still have a chance to replace
> equivalent phis in MergeMemNode::Ideal.

Is this a reliable way of finding a missed transformation opportunity?
Could it be that MergeMemNode::Ideal() runs before PhiNode::Identity()
has had a chance to run? that is the merge mem is first in the IGVN
queue and the Phi is next so the merge mem would first fail to optimize,
then the Phi would optimize, cause the merge mem to be enqueued again
and this time be properly optimized.

Roland.