Reapply "Fix negativity check in codegen" (#59986)

This partially reverts commit f8c7b22.

Author: yuyichao

src/codegen.cpp

@@ -4137,7 +4137,7 @@ static bool emit_builtin_call(jl_codectx_t &ctx, jl_cgval_t *ret, jl_value_t *f,
         if (argv[2].constant) {
             if (!jl_is_long(argv[2].constant))
                 return false;
-            size_t nel = jl_unbox_long(argv[2].constant);
+            auto nel = jl_unbox_long(argv[2].constant);
             if (nel < 0)
                 return false;
             *ret = emit_const_len_memorynew(ctx, typ, nel, inst);