[MDA] Don't be quite as conservative for noalias functions

If we encounter a noalias call that alias analysis can't analyse, we can fall down into the generic call handling rather than giving up entirely. I noticed this while reading through the code for another purpose.

I can't seem to write a test case which changes; that sorta makes sense given any test case would have to be an inconsistency in AA. Suggestions welcome.

Differential Revision: http://reviews.llvm.org/D15825

llvm-svn: 256802