This adds handling of {IrOpcode::kStringCharCodeAt} nodes to the escape
status analysis. Such uses are treated as escaping for now until we add
dedicated handling to the escape analysis reducer.

R=bmeurer@chromium.org
BUG=chromium:649492

Review URL: https://codereview.chromium.org/2363063002 .

Cr-Commit-Position: refs/heads/master@{#39660}