This preserves the original shared code of the underlying function when
bytecode is provided. The method in question should only ensure bytecode
is present, but should avoid switching compilation tiers of the given
function. It might be that the function was fast-tracked to baseline by
inlining without going through the interpreted tier first.

R=rmcilroy@chromium.org
TEST=mjsunit/regress/regress-crbug-635923
BUG=chromium:635923

Review-Url: https://codereview.chromium.org/2278543002
Cr-Commit-Position: refs/heads/master@{#38866}