[PATCH] don't unroll loops with zero trip count; fix for PR21409

[hfinkel at anl.gov]

>>! In D6200#5, @spatel wrote:
> This change actually is in the runtime unroller...why we're getting here for a compile-time constant loop is another bug?

The runtime unroller also does partial unrolling of large-trip-count loops. So we're probably in the right place (UINT_MAX is certainly above the full-unrolling limit).

http://reviews.llvm.org/D6200