fix(loop/loopform): 専用preheader作成でValueId定義エラー解消 (WIP)

**問題**: NYASH_LOOPFORM_PHI_V2=1 でValueId未定義エラー発生
- emit_preheader()が既存blockに命令追加 → forward reference
- if-then内のloopでpreheader_id = current_block()が誤動作

**解決策**: LLVM canonical loop form準拠
- 専用preheaderブロック作成 (before_loop → preheader → header)
- 変数スナップショットをnew_block()前に取得
- emit_jump(preheader)で明示的分離

**成果**:
✅ ValueId定義エラー完全解消 (ValueId(10) etc.)
✅ 詳細デバッグ出力追加 (variable_map追跡)
❌ 新しい型エラー発生 (要調査: variable_map不整合)

**デバッグ出力**:
- before_loop_id + variable_map size
- Block IDs (preheader/header/body/latch/exit)
- 変数ごとのparam判定 + iteration count

**次のステップ**: variable_map変更点特定
- before_loop: args=ValueId(0)
- prepare_structure: args=ValueId(1) ← なぜ変わる？

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/mir/loop_builder.rs

@@ -137,26 +137,46 @@ impl<'a> LoopBuilder<'a> {
         condition: ASTNode,
         body: Vec<ASTNode>,
     ) -> Result<ValueId, String> {
-        // Create loop structure blocks
-        let preheader_id = self.current_block()?;
+        // Create loop structure blocks following LLVM canonical form
+        // We need a dedicated preheader block to materialize loop entry copies
+        let before_loop_id = self.current_block()?;
+
+        // Capture variable snapshot BEFORE creating new blocks (at loop entry point)
+        let current_vars = self.get_current_variable_map();
+
+        if std::env::var("NYASH_LOOPFORM_DEBUG").is_ok() {
+            eprintln!("[loopform] before_loop_id={:?}, variable_map size={}",
+                     before_loop_id, current_vars.len());
+            for (name, value) in &current_vars {
+                eprintln!("  {} -> {:?}", name, value);
+            }
+        }
+
+        let preheader_id = self.new_block();
         let header_id = self.new_block();
         let body_id = self.new_block();
         let latch_id = self.new_block();
         let exit_id = self.new_block();
 
+        // Jump from current block to preheader
+        self.emit_jump(preheader_id)?;
+
         // Initialize LoopFormBuilder with preheader and header blocks
         let mut loopform = LoopFormBuilder::new(preheader_id, header_id);
 
-        // Capture current variable map snapshot at preheader
-        let current_vars = self.get_current_variable_map();
-
         // Pass 1: Prepare structure (allocate all ValueIds upfront)
         if std::env::var("NYASH_LOOPFORM_DEBUG").is_ok() {
-            eprintln!("[loopform] variable_map at loop entry:");
+            eprintln!("[loopform] Block IDs: preheader={:?}, header={:?}, body={:?}, latch={:?}, exit={:?}",
+                     preheader_id, header_id, body_id, latch_id, exit_id);
+            eprintln!("[loopform] variable_map at loop entry (size={}):", current_vars.len());
+            let mut loop_count = 0;
             for (name, value) in &current_vars {
+                loop_count += 1;
+                eprintln!("  [{}] {} -> {:?}", loop_count, name, value);
                 let is_param = self.is_parameter(name);
-                eprintln!("  {} -> {:?} (param={})", name, value, is_param);
+                eprintln!("    param={}", is_param);
             }
+            eprintln!("[loopform] iterated {} times", loop_count);
         }
         loopform.prepare_structure(self, &current_vars)?;