The transformation to add an OpPhi synonym (issue: #3592 , PR: #3701 )
accepts a mapping from the predecessors of a block to ids that must
all be synonymous. If an id corresponds to a predecessor that is a
dead block (it will never be executed), this rule can be relaxed and
we could accept any available id of the right type.

After the discussion: we could actually make a new transformation for
replacing ids in OpPhi instructions.