[AArch64] 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 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/rL332534
...because that change wasn't enough.

llvm-svn: 332636