[CFLAA] Be more conservative with values we haven't seen.

There were issues with simply reporting AttrUnknown on
previously-unknown values in CFLAnders. So, we now act *entirely*
conservatively for values we haven't seen before. As in the prior patch
(r277362), writing a lit test for this isn't exactly trivial. If someone
wants a test badly, I'm willing to try to write one.

Patch by Jia Chen.

Differential Revision: https://reviews.llvm.org/D23077

llvm-svn: 277533