Minor bugfix in AAResults::getModRefInfo.

Also removed a few redundant `else`s.

Bug was found by a test I wrote for MemorySSA (in review at
http://reviews.llvm.org/D7864; shiny update coming soon). So, assuming
that lands at some point, this should be covered by that. If anyone
feels this deserves its own explicit test case, please let me know.
I'll write one.

llvm-svn: 259179