Places that aren't testing for sign should not use the more expensive
function; it's just wasteful, not to mention being a cognitive load
for readers who may know what isinf() is but not is_infinite().

As things stand, we actually don't need is_infinite() anyplace except
float4out/float8out, which means it could potentially go away altogether
after the changes I proposed in <13178.1538794717@sss.pgh.pa.us>.