[APFloat] Fix getExactInverse for DoubleAPFloat
Some background: getExactInverse()'s callers expect that
the result is not subnormal.
DoubleAPFloat implemented getExactInverse() by going through
semPPCDoubleDoubleLegacy.
This means that numbers like 0x1p1022 which would have a normal inverse
in semPPCDoubleDouble would not in semPPCDoubleDoubleLegacy.
This commit refactors the logic into a single method on APFloat which
uses getExactLog2Abs() and scalbn() to calculate the inverse without
having to compute a reciprocal and test if it is inexact.
This approach works for both IEEEFloat and DoubleAPFloat.
