[OPTIM] make the equal_bits function work with signed ints
[OPTIM] make the equal_bits function work with signed ints

Since equal_bits() is only fed with signed ints and its result is used
as a signed it, we can save a few cycles by converting its arguments and
result to signed ints. Benchmarks show a 2 cycles reduction in the lookup
time on 4 bytes memory blocks (from 69 to 67 with 120k entries).

Note that due to many suboptimisations caused by gcc which constantly
copies data to register it never uses, we're constrained by the type
of optimisations we can perform there. quick code review show that at
least 3-4 cycles could be shaved off these functions.

sociomantic-tsunami/ebtree,sociomantic-tsunami/ebtree,haproxy/ebtree