[InstCombine] add nnan tests; NFC

As suggested in D37427, we could have a value tracking function and folds that use
it to simplify these cases. 


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@312578 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed