ART: Speed up HGraph::PopulateIrreducibleRecursive

Populating an irreducible loop can potentially traverse all possible
paths through the HGraph, leading to an exponential algorithm.
This patch adds a bit vector of nodes whose membership in the loop
has been decided and need not be revisited again.

Bug: 27856014
Change-Id: I3696f08c846e6f40e5de44cb771811bac7e3e08a