XPath: Only recognize numeric constant expressions
XPath: Only recognize numeric constant expressions

Numeric and boolean constant expressions in filters are different in that
to evaluate numeric expressions we need a sorted order, but to evaluate
boolean expressions we don't. The previously implemented optimization adds
an extra sorting step for constant boolean filters that will be more expensive
than redundant computations.

Since constant booleans are sort of an edge case, don't do this optimization.
This allows us to simplify apply_predicate_const to only handle numbers.

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