Auto merge of #146829 - cjgillot:jump-threading-loop-dominator, r=dianqk

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.

Author: bors