Avoid unnecessary descents into nested boxes with floats.

LayoutTests/fast/block/float/float-mark-descendants-for-layout-crash.html

+<style>
+.c7 { float: right; }
+.c8:nth-last-of-type(2n+1) { float: right;}
+</style>
+<script>
+var nodes = Array();
+function reference(domNode) {this.domNode = domNode;} function walk(a, currentPrefix, index, domNode) {if(domNode == null) return; newPrefix = currentPrefix + "_" + index;a[newPrefix] = new reference(domNode);} function clear() {var a = new Array();walk(a, "", 0, document.documentElement);for(key in a) {a[key].domNode.parentNode.removeChild(a[key].domNode);}}
+function boom() {
+    try { nodes[0] = document.createElement('meter'); } catch(e) {}
+    try { nodes[0].setAttribute('class', 'c7'); } catch(e) {}
+    try { document.documentElement.appendChild(nodes[0]); } catch(e) {}
+    try { nodes[58] = document.createElement('br'); } catch(e) {}
+    try { document.documentElement.appendChild(nodes[58]); } catch(e) {}
+    try { nodes[63] = document.createElement('h1'); } catch(e) {}
+    try { nodes[63].setAttribute('class', 'c8'); } catch(e) {}
+    try { document.documentElement.appendChild(nodes[63]); } catch(e) {}
+    try { nodes[66] = document.createElement('iframe'); } catch(e) {}
+    try { nodes[66].setAttribute('srcdoc', ''); } catch(e) {}
+    try { document.documentElement.appendChild(nodes[66]); } catch(e) {}
+    try { nodes[72] = document.createElement('sup'); } catch(e) {}
+    try { nodes[72].setAttribute('class', 'c7'); } catch(e) {}
+    try { document.documentElement.appendChild(nodes[72]); } catch(e) {}
+    setTimeout('clear();', 700);
+}
+window.onload = boom;
+</script>
+