has_eq_restriction_on_column() walked expression trees at prepare time to
find binary_operators with op==EQ that mention a given column on the LHS.
Its only caller is ORDER BY validation in select_statement, which checks
that clustering columns without an explicit ordering have an EQ restriction.
Replace the 50-line expression-walking free function with a precomputed
unordered_set<const column_definition*> (_columns_with_eq) populated during
the main predicate loop in analyze_statement_restrictions. For single-column
EQ predicates the column is taken from on_column; for multi-column EQ like
(ck1, ck2) = (1, 2), all columns in on_clustering_key_prefix are included.
The member function becomes a single set::contains() call.
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