[CFLAA] Make CFLAnders more conservative with new Values.

Currently, CFLAnders assumes that values it hasn't seen don't alias
anything. This patch fixes that. Given that the only way for this to
happen is to query AA, rely on specific transformations happening, then
query AA again (looking for a specific set of queries), lit testing is a
bit difficult. If someone really wants a test, I'm happy to add one.

Patch by Jia Chen.

Differential Revision: https://reviews.llvm.org/D22981

llvm-svn: 277362