[tree-ssa] Remove useless null pointer checks

[Andrew Haley]

Jeff Sturm writes:
 > 
 > A related question came up just the other day on the java list.
 > There are often-called functions (e.g. allocators) in libgcj that
 > cannot return null.  Is there a good way to tell the tree
 > optimizers about these, to eliminate even more useless null pointer
 > checks?

We could just invent an attribute, I suppose.  

We need a way to mark such an attribute in Java source.  We don't have
an attribute syntax and we want to stay Java compatible.  Stylized
comments, perhaps?

Andrew.