Revert r321751, "StructurizeCFG: Fix broken backedge detection"

It causes regressions in various OpenGL test suites.

Keep the test cases introduced by r321751 as XFAIL, and add a test case
for the regression.

Change-Id: I90b4cc354f68cebe5fcef1f2422dc8fe1c6d3514
Bugzilla: https://bugs.llvm.org/show_bug.cgi?id=36015

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@323355 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed