add test cases that demonstrate bad behavior

These are based on PR25016 and likely caused by a bug in
MachineCombiner's definition of improvesCriticalPathLen().


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@249249 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed