We need to characterize the types of dead (IrOpcode::kDead) nodes
introduced in compilation phases prior to representation selection.
Normally, a dead node isn't expected at the start of this phase. The
question is, which phase introduced the dead node and failed to
deal with it properly?

Bug: chromium:780658
Change-Id: Ief5b45480bb7d704a2d09dafd60b5d389e0fd42e
Reviewed-on: https://chromium-review.googlesource.com/765968Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49328}