This adds a new field to the header of every BytecodeArray which stores
the current nesting level up to which loop back edges are armed as OSR
points. The intention is to arm OSR points incrementally from outermost
to innermost until one fires (similar to OSR from FullCodegen).

R=rmcilroy@chromium.org
BUG=v8:4764

Review-Url: https://codereview.chromium.org/2172583002
Cr-Commit-Position: refs/heads/master@{#38017}