[DAG] add test to show FMF mismatch between IR and DAG; NFC

D45710 proposes to change this, but we have no test coverage 
for the first step in this process.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@331271 91177308-0d34-0410-b5e6-96231b3b80d8