What information exactly does an instruction cache store?
It literally stores lines of machine code from program memory (aka the entire instruction you line in your original post.

The fact you even discuss "storing all possible op codes in cache" points to a deeper misunderstanding. Talking about storing all possible op codes in cache (or any memory for that matter) has no meaning. All the possible opcodes that the processor can run are hard-wired into the logic circuitry of the processor. They aren't "stored" anywhere.