[ARM] preserve test intent by removing undef

We need to clean up the DAG floating-point undef logic.
This process is similar to how we handled integer undef
logic in https://reviews.llvm.org/D43141.

And as we did there, I'm trying to reduce the patch by
changing tests that would probably become meaningless
once we correct FP undef folding.

Follow-up to:
https://reviews.llvm.org/rL332538
...because that change wasn't enough.

llvm-svn: 332637