Saxon XSLT and XQuery Processor

The comparison A is B may succeed when A is an element,
text, comment, or PI node, and B is an attribute or
namespace node, if the two nodes happen to have the
same offset in their respective data structures.

This bug has been present since the introduction of the
TinyTree, it is also present in 6.5.3. (Although the
"is" operator is not available in XSLT 1.0, the same
code is used internally for evaluating a union and for
eliminating duplicates).

Reported by Anton Lapounov [antonl@microsoft.com]

Circumvention: reverse the test (use B is A).

Test case added: expr90

Source code fixed.