Fix clippy warnings.

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

cranelift/codegen/src/regalloc/virtregs.rs

@@ -259,9 +259,9 @@ impl UFEntry {
     /// Decode a table entry.
     fn decode(x: i32) -> Self {
         if x < 0 {
-            UFEntry::Link(Value::from_u32((!x) as u32))
+            Self::Link(Value::from_u32((!x) as u32))
         } else {
-            UFEntry::Rank(x as u32)
+            Self::Rank(x as u32)
         }
     }