cranelift_codegen::souper_harvest: Move preopt out of Context, into clif-util

This allows for more flexibility of when/where to harvest LHS candidates. For
example, we could choose to harvest candidates that overlap with and supercede
our current preopt peepholes.

This commit also makes sure that we compute the CFG before running preopt, when
harvesting LHS candidates via `clif-util souper-harvest`.

cranelift/codegen/src/context.rs

@@ -457,10 +457,8 @@ impl Context {
     #[cfg(feature = "souper-harvest")]
     pub fn souper_harvest(
         &mut self,
-        isa: &dyn TargetIsa,
         out: &mut std::sync::mpsc::Sender<String>,
     ) -> CodegenResult<()> {
-        self.preopt(isa)?;
         do_souper_harvest(&self.func, out);
         Ok(())
     }