micro-optimize nbtcompare.c routines

Here's a patch that adjusts several routines in nbtcompare.c and related
files to use the branchless integer comparison functions added in commit
6b80394. It's probably unlikely this produces a measurable benefit (at
least I've been unable to find any in my admittedly-limited testing), but
in theory it should save a cycle here and there. I was hoping that this
would trim many lines of code, but maintaining the STRESS_SORT_INT_MIN
stuff eats up most of what we save.

Anyway, I don't feel too strongly about this patch, but I went to the
trouble of writing it, and so I figured I'd post it.

--
nathan