This changes the IR to no longer require single {IfSuccess} projection
nodes unless there is a corresponding {IfException} node that links the
potentially throwing call to an exception handler. This reduces graph
size as well as compilation time when exception handlers aren't present.

The new invariant for potentially throwing nodes is: Nodes that can
potentially throw either have both IfSuccess/IfException projections as
the only control uses and no direct control uses, or no projections at
all and solely direct control uses.

R=jarin@chromium.org

Change-Id: I3d9cd816d74ad5af13e0673da7ec7a98f1ecdc7e
Reviewed-on: https://chromium-review.googlesource.com/449715
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43814}