Precomputing the hash values allows us to perform more frequent
accesses to the hash table, thereby reaching higher throughputs.

We keep the old behaviour by default, since (1) we might confuse
users if they measured a speedup without changing anything in
the QHT implementation, and (2) benchmarking the hash function
"on line" is also valuable.

Before:
$ taskset -c 0 tests/qht-bench -n 1
 Throughput:        38.18 MT/s

After:
$ taskset -c 0 tests/qht-bench -n 1
 Throughput:        38.16 MT/s

After (with precomputing):
$ taskset -c 0 tests/qht-bench -n 1 -p
 Throughput:        50.87 MT/s

Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>