Revert of Remove obsolete JSFunction::IsOptimizable predicate. (patchset #1 id:1 of https://codereview.chromium.org/1150683002/)

Reason for revert:
Causes assertions to fire when serializing optimized code.

Original issue's description:
> Remove obsolete JSFunction::IsOptimizable predicate.
>
> This just delegates to SharedFunctionInfo::optimization_disabled and
> was primarily used for assertions. Removing it due to misleading name
> because already optimized functions reported being "non-optimizable".
>
> R=titzer@chromium.org
>
> Committed: https://crrev.com/181d7b85977eb752b19e1de902093783e31330ef
> Cr-Commit-Position: refs/heads/master@{#28551}

TBR=titzer@chromium.org,bmeurer@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/1148973005

Cr-Commit-Position: refs/heads/master@{#28554}