[ast-optimizer-branch] Simplification is pretty expensive

[Diego Novillo]

It should not be a surprise, but re-writing the trees is a very
slow proposition.  I've started tracking the ast branch with the
SPEC95 tester and the build times are embarrassing (bootstrap
times are 65% higher).

At least the SPEC scores don't seem to have been affected.  It's
time to start profiling.  Although much of the slowdown could be
due to silly implementation blunders, we may find opportunities
in transformations that happen after simplification:

- The current SSA algorithms are naive.  There are several
  alternatives that could be used.

- Maybe we can lobotomize the tree->rtl pass now that it doesn't
  have to deal with complex trees (granted this is only true for
  the C front end now).


Diego.