Make FunctionInfo's assignment operator argument const

[dschuff]

Aside from the fact that there's no need for this to be non-const and this is the usual way to write an assignment operator, this is also needed because of a recent change to std::pair (llvm/llvm-project#89652). This seems to be forcing pair to want the const version of the assignment operator of its members.

[dschuff]

Error log is at
https://logs.chromium.org/logs/emscripten-releases/buildbucket/cr-buildbucket/8741863354289051889/+/u/Build_Binaryen/stdout

/b/s/w/ir/x/w/install/include/c++/v1/__utility/pair.h:102:31: error: the parameter for this explicitly-defaulted copy assignment operator is const, but a member or base requires it to be non-const
  102 |   _LIBCPP_HIDE_FROM_ABI pair& operator=(const pair&)
      |                               ^
/b/s/w/ir/x/w/install/include/c++/v1/unordered_map:840:14: note: in instantiation of template class 'std::pair<const wasm::Name, wasm::(anonymous namespace)::FunctionInfo>' requested here
  840 |   value_type __cc_;
      |              ^
/b/s/w/ir/x/w/install/include/c++/v1/unordered_map:1730:13: note: in instantiation of template class 'std::__hash_value_type<wasm::Name, wasm::(anonymous namespace)::FunctionInfo>' requested here
 1730 |       .first->__get_value()
      |             ^
/b/s/w/ir/cache/builder/emscripten-releases/binaryen/src/passes/Inlining.cpp:190:10: note: in instantiation of member function 'std::unordered_map<wasm::Name, wasm::(anonymous namespace)::FunctionInfo>::operator[]' requested here
  190 |     infos[getFunction()->name].hasLoops = true;