[InstCombine] add test for fneg+fsub with nsz; NFC

There used to be a fold that would handle this case more generally,
but it was removed at rL73243 to fix PR4374:
https://bugs.llvm.org/show_bug.cgi?id=4374


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@329322 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed