[APInt] Remove accumulator initialization from tcMultiply and tcFullMultiply. NFCI (#88202)

The tcMultiplyPart routine has a flag that says whether to add to the
accumulator or overwrite it. By using the overwrite mode on the first
iteration we don't need to initialize the accumulator to zero.

Note, the initialization in tcFullMultiply was only initializing the
first rhsParts of dst. tcMultiplyPart always overwrites the rhsParts+1
part that just contains the last carry. The first write to each part of
dst past rhsParts is a carry write so that's how the upper part of dst
is initialized.