When merging effect phis, we currently do not try to merge information
for any variables that are merged by phis on the same control merge.

This CL implements a very limited version of merging map information
for nodes: If all the map information for all inputs of the phi
is the same, we assign that map information to the phi itself. This
is somewhat consistent with merging map information for a node:
there we also combine the information only if in all incoming
paths we have teh same set of maps.

In theory, we could union the sets of maps. Also, we could consider
merging other information (such as abstract fields).

This yields some small improvement (~2%) on the Octane deltablue
benchmark.

Bug: v8:5267
Change-Id: I9447732e3f0d06eb44632db492782fa35529f9fd
Reviewed-on: https://chromium-review.googlesource.com/622792
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47475}