The analysis phase used to skip TypeGuard nodes, which are
normally re-introduced by the reduction phase. However, phi nodes
are created during the analysis phase already, and so it could happen
that a phi input skips a TypeGuard.

This CL solves the problem by not removing TypeGuard nodes in the first
place, but only forwarding the VirtualObject. This is analogous to how
we already treat FinishRegion nodes, which are similar in that they are
a renaming too.

Bug: chromium:741225
Change-Id: Icf8aa2d40a30d89788d875b37b9986111f9c966f
Reviewed-on: https://chromium-review.googlesource.com/958442
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51863}