[CFLAA] Add an initial CFLAnders implementation.

This adds an incomplete anders-style implementation for CFLAA. It's
incomplete in that it's missing interprocedural analysis, attrs
handling, etc. and that it needs more tests. More tests and features
will be added in future commits.

Patch by Jia Chen.

Differential Revision: https://reviews.llvm.org/D22291

llvm-svn: 275602