Don't tail-duplicate blocks that contain convergent instructions.

Summary:
Convergent instrs shouldn't be made control-dependent on other values,
but this is basically the whole point of tail duplication.  So just bail
if we see a convergent instruction.

Reviewers: iteratee

Subscribers: jholewinski, jhen, hfinkel, tra, jingyue, llvm-commits

Differential Revision: http://reviews.llvm.org/D17320

llvm-svn: 261540