[SelectionDAG] fold 'fneg undef' to undef

This is extracted from the original draft of D61419 with some additional tests.
We don't currently get this in IR (it's conservatively turned into a NaN),
but presumably that'll get updated as we add real IR support for 'fneg'
rather than 'fsub -0.0, x'.

The x86-32 run shows the following, and I haven't looked further to see why,
but that seems to be independent:
  Legalizing: t1: f32 = undef
  Trying to expand node
  Creating fp constant: t4: f32 = ConstantFP<0.000000e+00>

Differential Revision: https://reviews.llvm.org/D61516

llvm-svn: 360296