Convert another IRExpr transformer to new API.
Micro-optimise: avoid double-testing entry expressions.  If we came in
on entry A, we can't also have come in on entry B.  Reflect this in
BDD normalisation rules.
Make sure that when we parse a BDD ordering we set the nextRank hash
up correctly.  Otherwise, we end up re-using BDD ranks, which is very
bad.
Extra debug for phase 0 canonicalisation.