Remove a local variable from pop_1st_bit()

Remove the 'b' uint32_t local variable.
Optimized assembly is more or less the same
(one 'mov' instruction less), but now it is
written in a way more similar to the final assembly
flow so it should be easier for compiler to optimize.

Also guarantee that BitTable[] is always aligned.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>