Optimize should probably behave as much like `spirv-opt` as possible.

[bradgrantham-lunarg]

Most importantly, if optimize doesn't produce a new output file, it should exit with a non-zero result code. If the output file wouldn't be usable, optimize should probably not produce output and should exit with a non-zero result code.

Here are some specifics.

• If the source file wasn't trimmed, just copy it and succeed, or fail.
• If the source file didn't have a compression format supported by optimize, report an error and exit with failure code rather than silently succeed and don't produce output.
• If the source file couldn't be optimized any further, just copy it and succeed, or fail.