Adding an --emit-opt-clif CLI option

[dimitris-aspetakis]

The option --emit-opt-clif follows the --emit-clif implementation from wasmtime's CLI, but the output comes with the egraph optimizations applied. It has been helpful for work on #6260. From discussions in zulip, it seemed more generally desirable.

• I consider some small changes in crates/cranelift/src/compiler.rs to be a bit "hacky", probably due to my inexperience with the borrow checker...
• Maybe there are some CLI error messages you would like introduced (e.g. if the paths for --emit-clif and --emit-opt-clif are the same) — I haven't created any since I'm not even sure about the CLI interface you would prefer.
• The current state does not include support for the wasmtime explorer.

[github-actions]

Label Messager: wasmtime:config

It looks like you are changing Wasmtime's configuration options. Make sure to
complete this check list:

• If you added a new Config method, you wrote extensive documentation for
  it.

  Our documentation should be of the following form:

  Short, simple summary sentence.
  
  More details. These details can be multiple paragraphs. There should be
  information about not just the method, but its parameters and results as
  well.
  
  Is this method fallible? If so, when can it return an error?
  
  Can this method panic? If so, when does it panic?
  
  # Example
  
  Optional example here.
  

• If you added a new Config method, or modified an existing one, you
  ensured that this configuration is exercised by the fuzz targets.

  For example, if you expose a new strategy for allocating the next instance
  slot inside the pooling allocator, you should ensure that at least one of our
  fuzz targets exercises that new strategy.

  Often, all that is required of you is to ensure that there is a knob for this
  configuration option in wasmtime_fuzzing::Config (or one
  of its nested structs).

  Rarely, this may require authoring a new fuzz target to specifically test this
  configuration. See our docs on fuzzing for more details.

• If you are enabling a configuration option by default, make sure that it
  has been fuzzed for at least two weeks before turning it on by default.

---

To modify this label's message, edit the .github/label-messager/wasmtime-config.md file.

To add new label messages or remove existing label messages, edit the
.github/label-messager.json configuration file.

Learn more.

[alexcrichton]

Thanks for this! As a possible alternative though, what do you think about not adding a new option but redefining --emit-clif as this option instead? That way optimized clif would be emitted by deafult and non-optimized clif could be emitted with -C opt-level=0