Now we create merges, effect phis and phis eagerly. This should make it
easier to add loop support in future.

Bug: v8:5267
Change-Id: I5324aec323feff581f5b34235a0b3d3b8987127c
Reviewed-on: https://chromium-review.googlesource.com/637834
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47651}