Stop populating irreducible loop at header

Recent CL modified the (previously exponential) algorithm for
populating irreducible loops so as to not visit blocks multiple times.
However, the new condition for an early exit did not take into account
that it is not necessary to visit predecessors of the loop header.

The algorithm remains correct but does unnecessary work.

Bug: 27856014
Change-Id: If72f8be82e79838f0dd9678082a3bbaabb51e43b
(cherry picked from commit 5a620590)