[SimpleLoopUnswitch] Add a test case exposing a bug

[SimpleLoopUnswitch] Add a test case exposing a bug

This test exposes a bug in SimpleLoopUnswitch that leads to a crash on
assert(SuccessorsCount > 1 && "Cannot unswitch a condition without multiple distinct successors!");
when SimpleLoopUnswitch considers unswitching of a loop by a switch with one successor.

Fix will be submitted soon.

Patch Author: Daniil Suchkov.

Reviewers: reames, asbirlea, skatkov
Reviewed By: skatkov
Subscribers: zzheng, llvm-commits
Differential Revision: https://reviews.llvm.org/D64403

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@365600 91177308-0d34-0410-b5e6-96231b3b80d8