InstCombine: Perform basic isnan combines on llvm.is.fpclass

is.fpclass(x, qnan|snan) -> fcmp uno x, 0.0
is.fpclass(nnan x, qnan|snan|other) -> is.fpclass(x, other)

Start porting the existing combines from llvm.amdgcn.class to the
generic intrinsic. Start with the ones which aren't dependent on the
FP mode.