If type checks in simplified lowering produced dead value (i.e., of
type Type::None()), we have only propagated deadness along value
edges. With this CL, we also insert an Unreachable node after every
effectful node that produces dead value.

This is more consistent with dead code elimination, which also inserts
unreachable nodes after effectful nodes with value output None.

Bug: chromium:884052
Change-Id: Idcb168461f05f1811b2c9c16ab8ff179b259fbd3
Reviewed-on: https://chromium-review.googlesource.com/1228125Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55987}