[RFC] Mixed results with O(1) PHI arg lookup

[Paolo Bonzini]

> Even in RTL or tree-level basic blocks without PHI node, we still have
> calls to execute_on_new_edge and execute_on_removing_edge, which are
> callbacks that are called upon adding or removing every edge.  An edge
> redirection counts as one add and one remove operation.

You could add a BB_HAS_ATTACHED_INFO flag on bb's with PHI nodes, and 
only call the hooks if it is set.  The insn-attrtab performance looks great.

Paolo