In graph building, use liveness information to only create Phis
for live registers when merging two environments, or when preparing
a loop header. Similarly, only create loop exit value nodes for
live registers when exiting a loop.

Bug: v8:6701
Change-Id: Id6ac6a5518e6f1762530d871e92a46f42397928b
Reviewed-on: https://chromium-review.googlesource.com/615173
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47389}