CSE optimization question

[Richard Kenner]

    Long term all the CSE path following cap should be zapped.  It's horrible
    code with dreadful compile-time performance characteristics.  We can get
    the same optimizations (in fact more optimizations) by doing value
    numbering on expressions as we convert into SSA -- the cost for doing this
    during the translation into SSA is unmeasurable.

I asked this before, but why isn't GCSE making that obsolete now? It isn't,
but I don't understand *why* it isn't.