ART: Fix dominance for irreducible loops

Computation of dominance was broken in the presence of irreducible
loops because the algorithm assumed that back edges are always
dominated by their respective headers and a fix-point iteration is
therefore unnecessary.

This is not true for irreducible loops, forcing us to revisit their
loop headers and all dependent blocks. This patch adds a fix-point
iteration if a back edge not dominated by its header is found.

Bug: 28611485
Change-Id: If84044e49d5b9c682949648033d2861628d7fe05