feat: add basic block reordering pass and improve codegen naming

- Implement `reorder_blocks` (BBR) pass using DFS to maximize
fallthroughs.
- Update x86_64 backend to use actual function names in call
instructions instead of generic IDs.
- Replace the GCD test case in main with an iterative factorial test
module.
- Remove redundant validation check at the end of the optimization
pipeline.

src/backend/x86_64.rs

@@ -540,10 +540,7 @@ impl<'a> X86Backend<'a> {
                 }
             }
             Instruction::Call {
-                dest,
-                func: target_id,
-                args,
-                ..
+                dest, func, args, ..
             } => {
                 let arg_regs = ["rdi", "rsi", "rdx", "rcx", "r8", "r9"];
                 for (i, (_, arg_op)) in args.iter().enumerate() {
@@ -559,7 +556,15 @@ impl<'a> X86Backend<'a> {
                         }
                     }
                 }
-                writeln!(&mut self.assembly, "    call function_{}", target_id.0).unwrap();
+
+                let function_name = self
+                    .module
+                    .functions
+                    .iter()
+                    .find_map(|f| (f.id == *func).then(|| f.name.clone()))
+                    .unwrap();
+
+                writeln!(&mut self.assembly, "    call {}", function_name).unwrap();
                 if args.len() > 6 {
                     let cleanup_size = (args.len() - 6) * 8;
                     writeln!(&mut self.assembly, "    addq ${}, %rsp", cleanup_size).unwrap();