[APFloat] Fix fusedMultiplyAdd when `this` equals to `Addend`

Up until now, the arguments to `fusedMultiplyAdd` are passed by
reference. We must save the `Addend` value on the beginning of the
function, before we modify `this`, as they may be the same reference.

To fix this, we now pass the `addend` parameter of `multiplySignificand`
by value (instead of by-ref), and have a default value of zero.

Fix PR44051.

Differential Revision: https://reviews.llvm.org/D70422