Phase 33-3: If/PHI MIR pattern matching + Select lowering (minimal patterns)

Implementation:
- Implement MIR pattern matching in if_select.rs (simple/local patterns)
- Add try_lower_if_to_joinir() entry point in lowering/mod.rs
- Create comprehensive integration tests (4/4 passing)

Pattern Support:
- Simple pattern: if cond { return 1 } else { return 2 }
  - Both blocks must have Return only (no instructions)
- Local pattern: if cond { x = a } else { x = b }; return x
  - Each branch has exactly 1 Copy instruction
  - Both branches jump to same merge block
  - Merge block Returns the assigned variable

Safety Mechanisms:
- Dev toggle: NYASH_JOINIR_IF_SELECT=1 required
- Function name filter: Only IfSelectTest.* functions
- Fallback: Returns None on pattern mismatch
- Zero breaking changes: Existing if_phi path untouched

Tests (4/4 PASS):
- test_if_select_simple_pattern
- test_if_select_local_pattern
- test_if_select_disabled_by_default
- test_if_select_wrong_function_name

Files:
- Modified: src/mir/join_ir/lowering/if_select.rs (pattern matching)
- Modified: src/mir/join_ir/lowering/mod.rs (entry point)
- New: src/tests/mir_joinir_if_select.rs (integration tests)
- Modified: src/tests/mod.rs (module registration)

Phase 33-3.2/3.3 (legacy removal) pending

🤖 Generated with Claude Code
Co-Authored-By: Claude <noreply@anthropic.com>

src/mir/join_ir/lowering/mod.rs

@@ -41,3 +41,61 @@ pub use skip_ws::lower_skip_ws_to_joinir;
 pub use stage1_using_resolver::lower_stage1_usingresolver_to_joinir;
 pub use stageb_body::lower_stageb_body_to_joinir;
 pub use stageb_funcscanner::lower_stageb_funcscanner_to_joinir;
+
+// Phase 33: If/Else → Select lowering entry point
+use crate::mir::join_ir::JoinInst;
+use crate::mir::{BasicBlockId, MirFunction};
+
+/// Phase 33-3: Try to lower if/else to JoinIR Select instruction
+///
+/// Scope:
+/// - Only applies to functions matching "IfSelectTest.*"
+/// - Requires NYASH_JOINIR_IF_SELECT=1 environment variable
+/// - Falls back to traditional if_phi on pattern mismatch
+///
+/// Returns Some(JoinInst::Select) if pattern matched, None otherwise.
+pub fn try_lower_if_to_joinir(
+    func: &MirFunction,
+    block_id: BasicBlockId,
+    debug: bool,
+) -> Option<JoinInst> {
+    // dev トグルチェック
+    if !crate::config::env::joinir_if_select_enabled() {
+        return None;
+    }
+
+    // 関数名で制限（IfSelectTest.* のみ）
+    if !func.signature.name.starts_with("IfSelectTest.") {
+        if debug {
+            eprintln!(
+                "[try_lower_if_to_joinir] skipping non-test function: {}",
+                func.signature.name
+            );
+        }
+        return None;
+    }
+
+    // if_select lowering を試行
+    let lowerer = if_select::IfSelectLowerer::new(debug);
+
+    if !lowerer.can_lower_to_select(func, block_id) {
+        if debug {
+            eprintln!(
+                "[try_lower_if_to_joinir] pattern not matched for {}",
+                func.signature.name
+            );
+        }
+        return None;
+    }
+
+    let result = lowerer.lower_if_to_select(func, block_id);
+
+    if result.is_some() && debug {
+        eprintln!(
+            "[try_lower_if_to_joinir] if_select lowering used for {}",
+            func.signature.name
+        );
+    }
+
+    result
+}