[turbofan] Remove hack for dead nodes from JSGenericLowering.
[turbofan] Remove hack for dead nodes from JSGenericLowering.

Now that the graph is being trimmed after generic lowering, we can drop
this workaround. The diamond will no longer confuse the scheduler.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/1191913002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#29081}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh