This makes sure OSR entry points can be added without having special
{OsrPoll} instructions in the bytecode stream. Eventually we might end
up merging back-branches and OSR polls together. Any loop header can be
used as an OSR entry point.

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

Review-Url: https://codereview.chromium.org/2329933003
Cr-Commit-Position: refs/heads/master@{#39373}