Investigate performance impact of rearranging "can combine backwards" bit

[hsivonen]

For characters that are their own decomposition, the least significant bit signifies "can combine backwards". As of Unicode 16, this information is also needed for complex decompositions, but the same bit was already taken, so the second-least-significant bit is used (by #4860).

Investigate the performance impact of flipping around the two bit allocations for complex decompositions and unifying the "can combine backwards" bit check.

[sffc]

Seems like something that would be beneficial to do in 2.0. Anyone can take this and @hsivonen has left enough of a trail. Perhaps @echeran