A restricted version of fast division logic for runtime divisors.

Turns out the most complex method works for all divisors (except for 1).

Seems to be a win when there's a multiply-keep-high-half
instruction available. We could make this the default implementation
of division, but it's restricted to small divisors and is slower for
32-bit numerators on my machine. Run test/performance/const_division.cpp
for performance numbers on your machine.
Merge branch 'master' of github.com:halide/Halide