[BUG] arm gcc blend codegen is awful

[DenisYaroshevskiy]

https://godbolt.org/z/zb9Ts8E8o

eve::wide<int> blend(eve::wide<int> x, eve::wide<int> y) {
  return eve::blend(x, y, eve::pattern<0, 1, 1, 0>);
}

seems like gcc can't handle the constexpr here well enough