Cranelift: Sign/zero-extend docs do not match implementation

The docs for `uextend` and friends mention

> If the input and output types are the same, this is a no-op.

When called with matching types, the verifier complains though:

https://github.com/bytecodealliance/wasmtime/blob/5fc01bafc7185b797f71c37352cbe66aa740893c/cranelift/codegen/src/verifier/mod.rs#L1471

(Whoops, this probably shouldn't be labeled as a bug ^^)