[LoopSimplify] don't separate nested loops with convergent calls

Summary:
When a loop has multiple backedges, loop simplification attempts to
separate them out into nested loops. This results in incorrect control
flow in the presence of some functions like a GPU barrier. This change
skips the transformation when such "convergent" function calls are
present in the loop body.

Reviewed By: nhaehnle

Differential Revision: https://reviews.llvm.org/D80078