util/set: Pull out loop-invariant computations

util/set: Pull out loop-invariant computations

Unfortunately GCC can't do this for us, probably because we call the key
comparison function which GCC can't prove won't modify arbitrary memory.
This is a pretty hot function, so do the optimization manually to be
sure the compiler will get it right.

While we're here, make the computation of the new probe address use a
single conditional subtract instead of a modulo, since we know that it
won't ever get as big as 2 * ht->size before the modulo. Modulos tend to
be pretty expensive operations.

shader-db compile time results for my database:

Difference at 95.0% confidence
	-2.24934 +/- 0.69897
	-0.516296% +/- 0.159993%
	(Student's t, pooled s = 0.983684)

Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>