[x86] preserve test intent by removing undef

[x86] 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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@332648 91177308-0d34-0410-b5e6-96231b3b80d8