Add convergent-removing bits to FunctionAttrs pass.

Summary:
Remove the convergent attribute on any functions which provably do not
contain or invoke any convergent functions.

After this change, we'll be able to modify clang to conservatively add
'convergent' to all functions when compiling CUDA.

Reviewers: jingyue, joker.eph

Subscribers: llvm-commits, tra, jhen, hfinkel, resistor, chandlerc, arsenm

Differential Revision: http://reviews.llvm.org/D17013