Simplifies the traversal of nodes in Scheduler::PrepareUses to
avoid having to carefully order stack traversal for pre/post
ordering visits. Instead simply pre visit when pushing a node
onto the stack, then post visit the node when popping it from
the stack and then visiting it's inputs. This keeps the same
invariants required, but reduces visit overhead.

In addition, move checking for CoupledControlEdges out of
Increment/DecrementUnscheduledUseCounts such that the
coupled control edge calculation only needs to be done once
per node, rather than once for every input of the node. Also
remove unecessary recursion from these functions.

All told, these optimizations reduce the PrepareUses overhead
by 40-50%.

BUG=v8:9684

Change-Id: I934523a732892a1f66d7e77f8d04e200169080f1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2863602
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74373}