Simplify flo:-inf.0 and flo:+inf.0.

Simplify flo:-inf.0 and flo:+inf.0.

We have put out a release now whose reader understands +inf.0/-inf.0,
so we should be able to use the notation.

Fixes bugs with incorrectly raising the divide-by-zero exception flag
(even if no trap), e.g.:

;|log1mexp-invalid/(4.9406564584124654e-324)| failed 1 sub-tests out of 1 in 0. seconds:
     assertion 3: value was 3 but expected an object eqv? to 1

Here 3 = (invalid-operation divide-by-zero), 1 = (invalid-operation)
[aarch64].