fix opt/8634

[Richard Kenner]

    can you explain this in greater detail? It is one pass over insns, so it
    should be linear, no?

Each pass, yes, but what I meant is suppose you have situation where a
particular construct causes a call to purge_addressof.  That construct is
used in some percentage of code.  So the number of occurrences is proportional
to the number of insns.  That's where the quadratic behavior comes from.