This adds tracking of the loop depth to the {BytecodeGenerator} in order
to statically determine the loop nesting level for {OsrPoll} bytecodes.

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

Review-Url: https://codereview.chromium.org/2176183002
Cr-Commit-Position: refs/heads/master@{#38064}