Fix clippy warnings.

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

cranelift/frontend/src/variable.rs

@@ -16,14 +16,14 @@ impl Variable {
     /// Create a new Variable with the given index.
     pub fn with_u32(index: u32) -> Self {
         debug_assert!(index < u32::MAX);
-        Variable(index)
+        Self(index)
     }
 }
 
 impl EntityRef for Variable {
     fn new(index: usize) -> Self {
         debug_assert!(index < (u32::MAX as usize));
-        Variable(index as u32)
+        Self(index as u32)
     }
 
     fn index(self) -> usize {