cranelift: Use `bugpoint` mutators in `cranelift-icache` fuzzer

👋 Hey,

I had this thought overnight and I'm not sure if there are any hidden downsides.

Both `cranelift-icache` and `clif-util bugpoint` change the input functions. `cranelift-icache` is a bit more random in its approach and `bugpoint` has some constraints to preserve. However `bugpoint` is quite developed and can mutate functions in a bunch of ways that `icache` can't, we could try to use those mutators in `icache` and check if it breaks the cache or not.

cc: @bjorn3 @bnjbvr @jameysharp