feat(phi): Phase 41-1 delete dead PHI code (147 lines)

Delete pure dead code identified by Task agent investigation:

**if_phi.rs (-99 lines)**
- merge_modified_at_merge_with (70 lines) - zero external callsites
  Superseded by PhiBuilderBox::generate_if_phis()
- merge_with_reset_at_merge_with (29 lines) - wrapper for above

**conservative.rs (-48 lines)**
- get_conservative_values (48 lines) - zero callsites
  Superseded by PhiBuilderBox::get_conservative_if_values()

Tests: Phase 40 5/5 PASS, conservative 3/3 PASS
No regression (14 pre-existing failures unchanged)

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

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

src/mir/phi_core/conservative.rs

@@ -84,55 +84,8 @@ impl ConservativeMerge {
         }
     }
 
-    /// Conservative フォールバック値取得
-    ///
-    /// # Returns
-    /// * `Some((then_v, else_v))` - 両ブランチの値（void emission 含む）
-    /// * `None` - どこにも定義されていない変数（スキップ）
-    ///
-    /// # Conservative Rules
-    /// 1. Both defined: use both values
-    /// 2. Only then: use then + void
-    /// 3. Only else: use void + else
-    /// 4. Neither: skip (return None)
-    pub fn get_conservative_values<F>(
-        &self,
-        name: &str,
-        pre_if: &BTreeMap<String, ValueId>, // Phase 25.1: BTreeMap化
-        then_end: &BTreeMap<String, ValueId>, // Phase 25.1: BTreeMap化
-        else_end_opt: &Option<BTreeMap<String, ValueId>>, // Phase 25.1: BTreeMap化
-        emit_void: F,
-    ) -> Option<(ValueId, ValueId)>
-    where
-        F: Fn() -> ValueId,
-    {
-        let pre_val_opt = pre_if.get(name).copied();
-
-        // Fallback to predecessor value if not defined in a branch
-        let then_v_opt = then_end.get(name).copied().or(pre_val_opt);
-        let else_v_opt = else_end_opt
-            .as_ref()
-            .and_then(|m| m.get(name).copied())
-            .or(pre_val_opt);
-
-        match (then_v_opt, else_v_opt) {
-            (Some(tv), Some(ev)) => Some((tv, ev)),
-            (Some(tv), None) => {
-                // Variable exists in then branch but not else or predecessor
-                // Emit a 'const void' instruction to represent undefined value
-                Some((tv, emit_void()))
-            }
-            (None, Some(ev)) => {
-                // Variable exists in else branch but not then or predecessor
-                // Emit a 'const void' instruction to represent undefined value
-                Some((emit_void(), ev))
-            }
-            (None, None) => {
-                // Variable doesn't exist anywhere - skip
-                None
-            }
-        }
-    }
+    // Phase 41-1削除済み（2025-11-29）
+    // - get_conservative_values (48行) - PhiBuilderBox::get_conservative_if_values()に置き換え済み
 
     /// Debug trace 出力（Conservative PHI生成の可視化）
     pub fn trace_if_enabled(