This patch only treats non-private symbols as valid feedback, thus
avoiding the need to switch to Oddballs for the feedback sentinels
and avoiding breaking the use of private own symbols.

Crankshaft will also optimize these symbol loads into a named load,
just as it does for string keyed loads with type feedback.

BUG=

Review URL: https://codereview.chromium.org/1415333003

Cr-Commit-Position: refs/heads/master@{#31496}