The MapGuard node sits in the effect chain as a hint for other
optimization passes that a certain value has a certain (set of)
map(s) guarded by checks on the control chain. This is useful
to learn from explicit control flow inserted for polymorphic
property accesses, and then used as part of the polymorphic
inlining.

This change improves the score on the Octane/DeltaBlue benchmark
by around 7-8% and the score on the Octane/Richards benchmark by
like 3% on average.

Bug: v8:5267
Change-Id: Id0b0b2c72e6a9342d5750a0d62cf6be6fb8c5916
Also-By: jarin@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/620586
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47417}