PR c++/89652
* constexpr.c (struct constexpr_ctx): Change save_exprs type from
hash_set<tree> to vec<tree>.
(cxx_eval_call_expression): Adjust for save_exprs being a vec instead
of hash_set.
(cxx_eval_loop_expr): Likewise. Truncate the vector after each
removal of SAVE_EXPRs from values.
(cxx_eval_constant_expression) <case SAVE_EXPR>: Call safe_push
method on save_exprs instead of add.