XPath: Only call apply_predicates if necessary

In some cases constant overhead on step evaluation is important - i.e. for
queries that evaluate a simple step in a predicate expression. Eliminating
a redundant function call thus can prove worthwhile.

This change makes some queries (e.g. //*[not(*)]) 4% faster.
XPath: Partially inline xpath_node_set_raw::push_back

Previously push_back implementation was too big to inline; now the common case
(no realloc) is small and realloc variant is explicitly marked as no-inline.

This is similar to xml_allocator::allocate_memory/allocate_memory_oob and
makes some XPath queries 5% faster.