voronoi: Make printing of negative NaNs consistent

The difference between printing of negative NaNs caused trouble with
the reference output hashes. We now always canonicalize to positive
NaNs.

git-svn-id: https://llvm.org/svn/llvm-project/test-suite/trunk@196186 91177308-0d34-0410-b5e6-96231b3b80d8