[LVI] Fix a latent bug in getValueAt

This routine was returning Undefined for most queries.  This was utterly wrong.  Amusingly, we do not appear to have any callers of this which are actually trying to exploit unreachable code or this would have broken the world.

A better approach would be to explicit describe the intersection of facts.  That's blocked behind http://reviews.llvm.org/D14476 and I wanted to fix the current bug.

llvm-svn: 259446