[Wasm Exceptions] Scan catch events in RemoveUnusedModuleElements

[kripken]

There was an existing test for this, it turns out.

Also refactor away some annoying repeated code in that pass. visitTry is
the only actual change.

Noticed by the exceptions fuzzer emscripten-core/emscripten#13485, e.g. on wasm-opt -O1 on

#include <stdio.h> // avoid iostream C++ code, just test libc++abi, not libc++
#include <stdint.h>

extern void refuel();
extern void checkRecursion();
extern bool getBoolean();

struct Class {
  Class();
  ~Class();
};

void func_0() {
  while (getBoolean()) {
  }
}

int main() {
  // func_0
  puts("calling func_0");
  refuel();
  try {
    func_0();
  } catch(...) {
    puts("main caught from func_0");
  }
  return 0;
}```

[aheejin]

Thanks! I forgot to do this...