Builtins::kReturnReceiver is used for the Symbol.iterator function on
iterators, and just returns the iterator itself. For example, for-of
or yield* with a generator will first call generator[Symbol.iterator](),
which simply returns the generator itself. Inlining this particular
builtin into TurboFan is trivial and avoids that call completely,
enabling more possibilities for LoadElimination and EscapeAnalysis
to get rid of even more checks in common generator code.

BUG=v8:6344,v8:6351,v8:6354
R=jgruber@chromium.org

Review-Url: https://codereview.chromium.org/2938683002
Cr-Commit-Position: refs/heads/master@{#45927}