The control edges in a TurboFan graph can form a cycle. To break this cycle in the int64-lowering we add special handling for loop nodes. Similar handling already exists for phi nodes and effectphi nodes, which breaks cycles formed by value edges and effect edges, respectively.

Review-Url: https://codereview.chromium.org/2511503002
Cr-Commit-Position: refs/heads/master@{#41071}