minifier: drop functions, even when used recursively

[KTibow]

This is related to parity with this Rollup optimization

I've been messing around with bundling Svelte, and an important learning is that Rollup makes more complex optimizations than you might expect. Some of these optimizations aren't even found in Terser and esbuild, but they'll be needed in OXC if Rolldown is going to compete with Rollup in bundle size.

function a() {}
function b() {
  if (Math.random() > 0.1) b();
}
function c() {
  if (Math.random() > 0.1) d();
}
function d() {
  if (Math.random() > 0.1) c();
}

should minify to 0 bytes.