[tree-ssa] Memory usage on the rise again

[law@redhat.com]

So I go away for 2 days and when I return, memory usage on Gerald's
testcase for the tree-ssa branch has risen 5% (both in terms of 
total memory allocated by the GC system and in terms of persistent
memory allocated by the GC system).

I would like to ask that folks start looking closely at their changes
to see how they impact the memory system.  It's frustrating to spend
considerable time to get a few percent back to have all the gains
disappear in a few days.

jeff