[PM/LoopUnswitch] Detect irreducible control flow within loops and skip...

[PM/LoopUnswitch] Detect irreducible control flow within loops and skip unswitching non-trivial edges.

Summary:
This fixes the bug pointed out in review with non-trivial unswitching.

This also provides a basis that should make it pretty easy to finish
fleshing out a routine to scan an entire function body for irreducible
control flow, but this patch remains minimal for disabling loop
unswitch.

Reviewers: sanjoy, fedor.sergeev

Subscribers: mcrosier, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D45754

llvm-svn: 330357