BGap: Improve performance at low orders of alpha'

BGap: Improve performance at low orders of alpha'

With this change, the old (good) performance at low orders
of alpha' is recovered in the new approach from commit
3567ef36 ("BGap: HUGE ~140x performance improvement at
higher alpha' orders").

A future patch to include the #if 'w' >= N conditionals
inside the [0-9]*ptJregs.h files themselves (like the current
4- and 5-pt files) will improve the (already good) situation even
more. But that needs to wait until I can write a script to add
them automatically...as doing it manually like for 4- and 5-pts
is impractical at higher pts.