(high_bits << PyLong_SHIFT) + low_bits with
  (high_bits << PyLong_SHIFT) | low_bits
in Objects/longobject.c.  Motivation:
 - shouldn't unnecessarily mix bit ops with arithmetic ops (style)
 - this pattern should be spelt the same way thoughout (consistency)
 - it's very very very slightly faster: no need to worry about
   carries to the high digit (nano-optimization).