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.