Fix clippy warnings.

This commit fixes the current set of (stable) clippy warnings in the repo.

cranelift/reader/src/isaspec.rs

@@ -25,7 +25,7 @@ pub enum IsaSpec {
 impl IsaSpec {
     /// If the `IsaSpec` contains exactly 1 `TargetIsa` we return a reference to it
     pub fn unique_isa(&self) -> Option<&dyn TargetIsa> {
-        if let IsaSpec::Some(ref isa_vec) = *self {
+        if let Self::Some(ref isa_vec) = *self {
             if isa_vec.len() == 1 {
                 return Some(&*isa_vec[0]);
             }