Add new ISD nodes: ISD::FMINNAN and ISD::FMAXNAN

Add new ISD nodes: ISD::FMINNAN and ISD::FMAXNAN

The intention of these is to be a corollary to ISD::FMINNUM/FMAXNUM,
differing only on how NaNs are treated. FMINNUM returns the non-NaN
input (when given one NaN and one non-NaN), FMINNAN returns the NaN
input instead.

This patch includes support for scalarizing, widening and splitting
vectors, but not expansion or softening. The reason is that these
should never be needed - FMINNAN nodes are only going to be created
in one place (SDAGBuilder::visitSelect) and there we'll check if the
node is legal or custom. I could preemptively add expand and soften
code, but I'm fairly opposed to adding code I can't test. It's bad
enough I can't create tests with this patch, but at least this code
will be exercised by the ARM and AArch64 backends fairly shortly.

llvm-svn: 244581