This ensures that all expressions that throw actually mark the current
environment as dead in the AstGraphBuilder. This prevents live ranges
from being unnecessarily increased by paths that don't fall-through.
Note that we can do that because Runtime::kThrowFoo never returns.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/1049203002

Cr-Commit-Position: refs/heads/master@{#27546}