XPath: Optimize [position()=expr] and [last()]

To get more benefits from constant predicate/filter optimization we rewrite
[position()=expr] predicates into [expr] for numeric expressions. Right now
the rewrite is only for entire expressions - it may be beneficial to split
complex expressions like [position()=constant and expr] into [constant][expr]
but that is more complicated.

last() does not depend on the node set contents so is "constant" as far as
our optimization is concerned so we can evaluate it once.

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