[turbofan] Handle StringCharCodeAt in escape analysis.

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}

src/compiler/escape-analysis.cc

@@ -795,6 +795,7 @@ bool EscapeStatusAnalysis::CheckUsesForEscape(Node* uses, Node* rep,
       case IrOpcode::kSelect:
       // TODO(mstarzinger): The following list of operators will eventually be
       // handled by the EscapeAnalysisReducer (similar to ObjectIsSmi).
+      case IrOpcode::kStringCharCodeAt:
       case IrOpcode::kObjectIsCallable:
       case IrOpcode::kObjectIsNumber:
       case IrOpcode::kObjectIsReceiver: