Why does Emsctipten use wasm-opt given the great optimization from LLVM?

[Shawdox]

Since Emscripten uses clang to generate object/wasm files at compile time (e.g. C) and uses wasm-opt to optimize the code at linking time, and since both clang and wasm-opt are given the same optimization level (e.g. O2), why do we still need to use wasm-opt to optimize the Wasm binary after optimizing on LLVM-IR?

1. Is there some optimization that can only be done on Wasm binary?
2. In addition to Wasm specificed passes, wasm-opt also provides some general passes, are these general passes not possible with LLVM's optimization?
3. For the same optimization level, which passes are used by clang and wasm-opt, and do any of these passes have the same functionality?

[kripken]

Closing as duplicate of WebAssembly/binaryen#7190