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/tests/mod.rs

@@ -20,6 +20,7 @@ pub mod mir_funcscanner_ssa;
 pub mod mir_funcscanner_trim_min;
 pub mod mir_joinir_funcscanner_append_defs; // Phase 27.14: FuncScannerBox._append_defs JoinIR変換
 pub mod mir_joinir_funcscanner_trim; // Phase 27.1: FuncScannerBox.trim JoinIR変換
+pub mod mir_joinir_if_select; // Phase 33-3: If/Else → Select lowering tests
 pub mod mir_joinir_min; // Phase 26-H: JoinIR型定義妥当性確認
 pub mod mir_joinir_skip_ws; // Phase 27.0: minimal_ssa_skip_ws JoinIR変換
 pub mod mir_joinir_stage1_using_resolver_min; // Phase 27.12: Stage1UsingResolverBox.resolve_for_source JoinIR変換