The existing `decodeDouble_2Int#` primop is rather inconvenient to use

(and in fact is not even used by `integer-gmp`) as the mantissa is split

into 3 components which would actually fit in an `Int64#` value.

However, `decodeDouble_Int64#` is to be used by the new `integer-gmp2`

re-implementation (see Trac #9281).

Moreover, `decodeDouble_2Int#` performs direct bit-wise operations on the

IEEE representation which can be replaced by a combination of the

portable standard C99 `scalbn(3)` and `frexp(3)` functions.