[tree-ssa] Speedup alias analysis [patch]

[Diego Novillo]

I forgot to add that with this patch, I can compile some of the
POOMA tests in a fraction of the time and memory that we used to.
For the files Cartesian.ii, FieldCentering.cmpl.ii and
LocalPatch.ii, we no longer need more than 1Gb of core to compile
and the compile times are reduced from 1361 secs to 551 secs.
Alias analysis in particular goes from 163 secs to 13 secs.


Diego.