Optimize RenderTable::colToEffCol() for tables without colspans

PerformanceTests/Layout/resources/large-table-with-collapsed-borders.js

+(function() {
+    function createElement(tag, parent, className, id) {
+        var el = document.createElement(tag);
+        el.className = className;
+        if (id)
+            el.id = id;
+        parent.appendChild(el);
+        return el;
+    }
+
+    function createTable(width, height, colspan) {
+        var table = createElement("table", document.body, "table");
+        for (var y = 0; y < height; ++y) {
+            var tr = createElement("tr", table, "tr");
+            for (var x = 0; x < width; ++x) {
+                var td = createElement("td", tr, "td");
+                if (colspan > 0 && x==10 && y==0)
+                    table.rows[y].cells[x].colSpan = colspan;
+            }
+        }
+        return table;
+    }
+
+    function createTestFunction(width, height, colspan) {
+        return function() {
+            var table = createTable(width, height, colspan);
+            table.clientHeight;
+        }
+    }
+
+    window.createTableTestFunction = createTestFunction;
+})();