only do clobber string computation if tracing

src/ion/fast_alloc.rs

@@ -445,15 +445,17 @@ fn allocate_block_insts<'a, F: Function>(
             state.func.is_branch(inst),
             alloc_idx
         );
-        let mut str = String::new();
-        for preg in clobbers {
-            if str.is_empty() {
-                str.push_str(&format!("{}", preg));
-            } else {
-                str.push_str(&format!(", {}", preg));
+        if trace_enabled!() {
+            let mut str = String::new();
+            for preg in clobbers {
+                if str.is_empty() {
+                    str.push_str(&format!("{}", preg));
+                } else {
+                    str.push_str(&format!(", {}", preg));
+                }
             }
+            trace!("Clobbers: {}", str);
         }
-        trace!("Clobbers: {}", str);
 
         // keep track of which pregs where allocated so we can clear them later on
         // TODO: wouldnt need this if we look up the inst a vreg was allocated at