Fix assertion in GetLoopUnrollInfo

The assertion added in https://review.skia.org/1236656 was too tight.
We can have unsigned integers in here too. We just treat them like ints
for figuring out if the loop will terminate.

Change-Id: Ia06ef0fb610397dfc8b6af71bcf3005927a07533
Bug: oss-fuzz:515515145
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1253476
Reviewed-by: Jorge Betancourt <jmbetancourt@google.com>
Commit-Queue: Jorge Betancourt <jmbetancourt@google.com>
Auto-Submit: Kaylee Lubick <kjlubick@google.com>
Commit-Queue: Kaylee Lubick <kjlubick@google.com>