Use if cfg!(...) instead of #[cfg] (#12889)

Minor style follow-up from #12841

Author: alexcrichton

cranelift/codegen/build.rs

@@ -223,20 +223,17 @@ fn run_compilation(compilation: &IsleCompilation) -> Result<(), Errors> {
         // threshold, because we cannot rely on rustc doing regalloc
         // on all of the local bindings to shrink the stack frame to a
         // reasonable size.
-        #[cfg(not(debug_assertions))]
-        {
+        if cfg!(debug_assertions) {
+            options.split_match_arms = true;
+            options.match_arm_split_threshold = Some(4);
+        } else {
             options.split_match_arms = std::env::var("CARGO_FEATURE_ISLE_SPLIT_MATCH").is_ok();
             if let Ok(value) = std::env::var("ISLE_SPLIT_MATCH_THRESHOLD") {
                 options.match_arm_split_threshold = Some(value.parse().unwrap_or_else(|err| {
                     panic!("invalid ISLE_SPLIT_MATCH_THRESHOLD value '{value}': {err}");
                 }));
             }
         }
-        #[cfg(debug_assertions)]
-        {
-            options.split_match_arms = true;
-            options.match_arm_split_threshold = Some(4);
-        }
 
         if let Ok(out_dir) = std::env::var("OUT_DIR") {
             options.prefixes.push(isle::codegen::Prefix {