SMT2 incremental back-end: don't use irep_full_hash

`irep_full_hash` would require also using `irep_full_eq`, but there is
no need to consider comments in this context. This fixes the unexpected
failures seen in runs like
https://github.com/diffblue/cbmc/actions/runs/5069698268/jobs/9103685508.

Author: tautschnig

src/solvers/smt2_incremental/convert_expr_to_smt.cpp

@@ -34,8 +34,7 @@
 ///   * avoiding the deeply nested call stacks associated with recursion.
 ///   * supporting wider scope for the conversion of specific types of `exprt`,
 ///     without inflating the parameter list / scope for all conversions.
-using sub_expression_mapt =
-  std::unordered_map<exprt, smt_termt, irep_full_hash>;
+using sub_expression_mapt = std::unordered_map<exprt, smt_termt, irep_hash>;
 
 /// \brief Converts operator expressions with 2 or more operands to terms
 ///   expressed as binary operator application.

src/solvers/smt2_incremental/type_size_mapping.h

@@ -15,7 +15,7 @@
 
 #include <unordered_map> // IWYU pragma: keep
 
-using type_size_mapt = std::unordered_map<typet, smt_termt, irep_full_hash>;
+using type_size_mapt = std::unordered_map<typet, smt_termt, irep_hash>;
 
 /// This function populates the (pointer) type -> size map.
 /// \param expression: the expression we're building the map for.