This way we don't have to deal with dead pre-OSR code in the graph and
risk optimizing the wrong code, especially we don't make optimistic
assumptions in the dead code that leaks into the OSR code (i.e. deopt
guards are in dead code, but the types propagate to OSR code via the
OsrValue type back propagation).

BUG=v8:4273
LOG=n
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/1215333005

Cr-Commit-Position: refs/heads/master@{#29478}