The compiler can generate a named access for o[x] if x is a compile time
constant that can be turned into a name using ToName (limited to
primitive x values, because other ToName invocations might be observable),
or the KeyedLoadIC/KeyedStoreIC have gather constant name feedback for x
(i.e. the access always goes to the same symbol).

R=jarin@chromium.org
BUG=v8:4470
LOG=n

Review URL: https://codereview.chromium.org/1414013004

Cr-Commit-Position: refs/heads/master@{#31703}