`wasm-opt -O <file>` should throw an error

[surma]

When I run wasm-opt -O3 myfile.wasm, I get no error and no output, but myfile.wasm stays the same. This lead me to believe that these files had already been properly optimized.

Then @kripken told me that I had to specify an output file like this: wasm-opt -O3 -o myfile2.wasm myfile.wasm.

I think my first command should error.

[kripken]

Ah, I saw this after I opened #1908

Options seem to be

• Emit a warning (what that PR does).
• Emit an error (but sometimes people do want to not output anything, like if just running --metrics).
• Do what llvm-opt does, which is always output (to stdout).
• Not change anything.

[surma]

Personally, I'd expect wasm-opt to overwrite the input file. That being said, all but the last option work for me, so I fully support your PR :)