[SelectionDAG] soften assertion when legalizing narrow vector FP ops
The test based on PR42010:
https://bugs.llvm.org/show_bug.cgi?id=42010
...may show an inaccuracy for PPC's target defs, but we should not be
so aggressive with an assert here. There's no telling what
out-of-tree targets look like.
This fixes an assertion when building the graphics/mesa-dri port for
PowerPC64.
Reported by: Mark Millard <marklmi26-fbsd@yahoo.com>
PR: 238082
MFC after: 3 days
b9ba849cf8