machinst x64: remove dead code and allow(dead_code) annotation;

The BranchTarget is always used as a label, so just use a plain
MachLabel in this case.

cranelift/codegen/src/machinst/buffer.rs

@@ -148,6 +148,7 @@ use crate::timing;
 use log::trace;
 use smallvec::SmallVec;
 use std::mem;
+use std::string::String;
 
 /// A buffer of output to be produced, fixed up, and then emitted to a CodeSink
 /// in bulk.
@@ -259,6 +260,11 @@ impl MachLabel {
     pub fn get(self) -> u32 {
         self.0
     }
+
+    /// Creates a string representing this label, for convenience.
+    pub fn to_string(&self) -> String {
+        format!("label{}", self.0)
+    }
 }
 
 /// A stack map extent, when creating a stack map.