JumpThreading: Avoid computing dominators to identify loop headers. JumpThreading tries to avoid threading through loop headers to avoid creating irreducible CFGs. However, computing dominators is expensive, and accounts up to 20 % of the runtime of the JumpThreading pass for some cases like serde. This PR proposes to approximate according to the post-order traversal order. We define a "maybe" loop header as a block which is visited after its predecessor in post-order.