We have fast-paths in identifier and string scanning that know that so far they have only handled one-byte input. In that case, we can add chars to the literal buffer assuming that it's still in one-byte state, avoiding 2 branches. This seemingly reduces identifier scanning from ~5% to ~4% on octane-codeload.

BUG=

Review-Url: https://codereview.chromium.org/2044233004
Cr-Commit-Position: refs/heads/master@{#36868}