Load elimination is running together with to dead code elimination, the
latter of which might eliminate allocations (in particular FinishRegion
nodes). These are treated as alias nodes by load elimination, and load
elimination does not immediatelly learn that a node has been disconnected.
This causes load elimination to access the inputs of dead code eliminated
nodes while resolving renames, which causes nullptr dereferences.

This CL modifies load elimination to not resolve to a nullptr alias but
simply stop before that.

Change-Id: If4cef061c7c0e25f353727c9e27f790439b0beb5
Bug: chromium:906406
Reviewed-on: https://chromium-review.googlesource.com/c/1346491
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57688}