[LoopRotate] Don't perform loop rotation if the loop header calls a convergent function.

Summary:
Calls to convergent functions can be duplicated, but only if the
duplicates are not control-flow dependent on any additional values.
Loop rotation doesn't meet the bar.

Reviewers: jingyue

Subscribers: mzolotukhin, llvm-commits, arsenm, joker.eph, resistor, tra, hfinkel, broune

Differential Revision: http://reviews.llvm.org/D17127

llvm-svn: 260729