Fix compiler warnings under emcc. NFC (#5104)
Fix compiler warnings under emcc. NFC (#5104)

These comparisons were showing up as warnings with the latest
emcc/clang.  I only noticed because I ran `ninja` to build everything
whereas we only test the `binaryen` target under emscripten normally.
WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen