feat(joinir): Phase 38 If-side PHI Level 1 deletion (90 lines, LOW safety)

Phase 38: Level 1 deletion complete (exceeded target by 22 lines)
- Deleted merge_modified_with_control (51 lines, dead code, 0 callsites)
- Deleted extract_assigned_var (39 lines, JoinIR AST lowering replacement)
- Updated callsites: if_form.rs (2), phi.rs (2) → replaced with None

File changes:
- if_phi.rs: 315 → 225 lines (90 lines, 28.6% reduction)
- Callsites updated: 4 sites (if_form.rs, phi.rs)

Technical achievements:
✅ JoinIR coverage verification (None replacement passes all tests)
✅ Dead code elimination (merge_modified_with_control 0 callsites)
✅ Staged deletion strategy validation (Phase 37 3-level plan works)

Test results:
✅ cargo build --release: Clean
✅ PHI tests: 58/58 PASS (no regression)
✅ JoinIR Frontend tests: 37/38 PASS (1 failure pre-existing test ordering)
✅ Full lib tests: 399-400/460 PASS (10-12 non-deterministic failures baseline)

Phase 35-38 cumulative: 605 lines deleted (430+107+90)
Phase 39+ potential: 415 lines (Level 2: 115, Level 3: 300)

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

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

src/mir/phi_core/if_phi.rs

@@ -34,45 +34,6 @@ pub fn infer_type_from_phi(
     None
 }
 
-/// Extract the assigned variable name from an AST fragment commonly used in
-/// if/else analysis. Same logic as builder::phi::extract_assigned_var.
-pub fn extract_assigned_var(ast: &ASTNode) -> Option<String> {
-    match ast {
-        ASTNode::Assignment { target, .. } => {
-            if let ASTNode::Variable { name, .. } = target.as_ref() {
-                Some(name.clone())
-            } else {
-                None
-            }
-        }
-        ASTNode::Program { statements, .. } => {
-            statements.last().and_then(|st| extract_assigned_var(st))
-        }
-        ASTNode::If {
-            then_body,
-            else_body,
-            ..
-        } => {
-            let then_prog = ASTNode::Program {
-                statements: then_body.clone(),
-                span: crate::ast::Span::unknown(),
-            };
-            let tvar = extract_assigned_var(&then_prog);
-            let evar = else_body.as_ref().and_then(|eb| {
-                let ep = ASTNode::Program {
-                    statements: eb.clone(),
-                    span: crate::ast::Span::unknown(),
-                };
-                extract_assigned_var(&ep)
-            });
-            match (tvar, evar) {
-                (Some(tv), Some(ev)) if tv == ev => Some(tv),
-                _ => None,
-            }
-        }
-        _ => None,
-    }
-}
 
 /// Collect all variable names that are assigned within the given AST subtree.
 /// Useful for computing PHI merge candidates across branches/blocks.
@@ -262,54 +223,3 @@ pub fn merge_with_reset_at_merge_with<O: PhiMergeOps>(
     )
 }
 
-/// Phase 25.1g: ControlForm-based wrapper for If PHI generation.
-/// This provides a thin adapter layer that accepts ControlForm and delegates
-/// to the existing merge_modified_at_merge_with implementation.
-///
-/// **Important**: This does NOT change any PHI generation logic - it merely
-/// provides a ControlForm-based entry point while preserving existing behavior.
-pub fn merge_modified_with_control<O: PhiMergeOps>(
-    ops: &mut O,
-    form: &crate::mir::control_form::ControlForm,
-    pre_if_snapshot: &BTreeMap<String, ValueId>,
-    then_map_end: &BTreeMap<String, ValueId>,
-    else_map_end_opt: &Option<BTreeMap<String, ValueId>>,
-    skip_var: Option<&str>,
-    // Existing implementation requires pred info, so we accept it as parameters
-    then_pred_opt: Option<crate::mir::BasicBlockId>,
-    else_pred_opt: Option<crate::mir::BasicBlockId>,
-) -> Result<(), String> {
-    use crate::mir::control_form::ControlKind;
-
-    // Only process If structures; silently succeed for other control kinds
-    let shape = match &form.kind {
-        ControlKind::If(shape) => shape,
-        _ => return Ok(()),
-    };
-
-    // Extract merge_bb from IfShape
-    let merge_bb = shape.merge_block;
-
-    // Log ControlForm usage when trace is enabled
-    let trace = std::env::var("NYASH_IF_TRACE").ok().as_deref() == Some("1");
-    if trace {
-        eprintln!(
-            "[if-phi/control-form] Using ControlForm wrapper: merge={:?} then={:?} else={:?}",
-            merge_bb, shape.then_block, shape.else_block
-        );
-    }
-
-    // Delegate to existing implementation - no behavioral changes
-    merge_modified_at_merge_with(
-        ops,
-        merge_bb,
-        shape.then_block,
-        shape.else_block.unwrap_or(shape.then_block),
-        then_pred_opt,
-        else_pred_opt,
-        pre_if_snapshot,
-        then_map_end,
-        else_map_end_opt,
-        skip_var,
-    )
-}