make SimpleXML boolean casts even faster

This is a follow up to PR #2855.

When we need to build our element's child tree (sxe_get_prop_hash) to
determine if it casts to true/false, we can get away with bailing after
finding the first element.

In my small Magento siege tests these result in significant
improvements:

Before:
Transactions:               1551 hits
Transaction rate:           51.79 trans/sec
Lowest response time:       97ms
Callgrind:                  http://i.imgur.com/npxMo0q.png

After:
Transactions:               1730 hits
Transaction rate:           58.43 trans/sec
Lowest response time:       89ms
Callgrind:                  http://i.imgur.com/Dgojb9Y.png