Improve the part combiner's silence analysis

See the improvement in regtest part-combine-mmrest-shared.ly. A rest in a measure by itself (because a whole-measure rest in the other part is properly omitted) was previously vertically shifted, but now it is not.

This result is now consistent with part-combine-silence-mixed.ly, which has the same combination of rest and whole-measure rest, but in the first measure.

This code change may seem out of proportion to its effect, but it is also a prerequisite to fixing #4599.