Prevent division by 0.
Prevent division by 0.

When we try to estimate number of potentially removed instructions in
loop unroller, we analyze first N iterations and then scale the
computed number by TripCount/N. We should bail out early if N is 0.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@228988 91177308-0d34-0410-b5e6-96231b3b80d8

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