Instead of doing a full function body traversal we collect return expressions and mark them after function parsing.

And since we rewrite do-expressions so that the result is explicitly assigned to a result variable the statements marking will never hit so I removed it from the AST.

BUG=v8:4698
LOG=N

Review URL: https://codereview.chromium.org/1693523002

Cr-Commit-Position: refs/heads/master@{#33911}