[APFloat][ADT] Fix sign handling logic for FMA results that truncate to zero.

This patch adds a check for underflow when truncating results back to lower
precision at the end of an FMA. The additional sign handling logic in
APFloat::fusedMultiplyAdd should only be performed when the result of the
addition step of the FMA (in full precision) is exactly zero, not when the
result underflows to zero.

Unit tests for this case and related signed zero FMA results are included.

Fixes <rdar://problem/18925551>.

llvm-svn: 225123