This turns the previous compile time constant IC::kMaxPolymorphicMapCount
into a runtime flag --max-polymorphic-map-count (defaults to 4 still),
which makes it easier to play with different number of maps in POLYMORPHIC
state.

Bug: v8:8765
Change-Id: I86c04d9b2d8be21a3bc6b31f70b0bbc359067467
Reviewed-on: https://chromium-review.googlesource.com/c/1456097Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59398}