htable: avoid branch in calculating perfect bit.

Final results of tools/speed/10000000 (10 runs) shows a slight
slowdown in some tests, but it makes an empty htable smaller.

-Initial delete all: 96-98(96.4+/-0.66) ns
+Initial delete all: 97-99(98.2+/-0.75) ns
-Initial re-inserting: 117-124(121.4+/-1.9) ns
+Initial re-inserting: 124-131(126.4+/-2.4) ns
-Adding (a different) half: 49-50(49.3+/-0.46) ns
+Adding (a different) half: 50-52(51.2+/-0.75) ns

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>