This reverts the following commits to fix a Node.js regression:
* 5529430d "[compiler] Consistently use Ignition+TurboFan for lexical variables."
* 78691367 "[compiler] Improve let+const decision in AstNumbering."

R=adamk@chromium.org, bmeurer@chromium.org, hablich@chromium.org
BUG=
NOTRY=true

Review-Url: https://codereview.chromium.org/2647523002
Cr-Commit-Position: refs/heads/master@{#42484}