Allow unused registers to be output to reflection

When compiling, the compiler will detect unused registers and kick them. This is great to be able to check what is actually used by the shader, but unfortunately that also prevents us from having stable shader reflection. If the shader artist connects a variable that is now compiled out, it would break their workflow. And if another variable is now not compiled out they'll have to connect it again. It would be great if O0 or a flag to avoid stripping this is available. Because right now we rely on manually tokenizing and parsing the entire file.

Start by tracing the compiler's unused-register elimination and shader reflection paths described in the issue, then identify how an O0 mode or preservation flag would affect both. Done means reflection can expose unused registers consistently without requiring manual tokenization and parsing of the source file.