allow u64 promote to i64

vlib/v/checker/check_types.v

@@ -741,7 +741,7 @@ fn (c &Checker) promote_num(left_type ast.Type, right_type ast.Type) ast.Type {
 		return type_lo // conversion unsigned u16 -> signed if signed type is larger
 	} else if idx_hi == ast.u32_type_idx && idx_lo > ast.int_type_idx {
 		return type_lo // conversion unsigned u32 -> signed if signed type is larger
-	} else if idx_hi == ast.u64_type_idx && idx_lo > ast.i64_type_idx {
+	} else if idx_hi == ast.u64_type_idx && idx_lo >= ast.i64_type_idx {
 		return type_lo // conversion unsigned u64 -> signed if signed type is larger
 	} else if idx_hi == ast.usize_type_idx && idx_lo > ast.isize_type_idx {
 		return type_lo // conversion unsigned usize -> signed if signed type is larger