Fix rounding correctness of additional 64-bit addition operations

[mborland]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.3%. Comparing base (d6cdb4e) to head (78b08ef).
⚠️ Report is 12 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1048     +/-   ##
=========================================
- Coverage     98.4%   98.3%   -0.0%     
=========================================
  Files          251     251             
  Lines        16504   16479     -25     
  Branches      1854    1850      -4     
=========================================
- Hits         16224   16197     -27     
- Misses         280     282      +2     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal64_t.hpp	97.2% <100.0%> (-<0.1%)	⬇️
include/boost/decimal/decimal_fast64_t.hpp	97.9% <100.0%> (-<0.1%)	⬇️
include/boost/decimal/detail/add_impl.hpp	90.4% <ø> (-2.5%)	⬇️
include/boost/decimal/detail/cmath/fma.hpp	100.0% <100.0%> (ø)

... and 3 files with indirect coverage changes

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