Remove hir::Guard

Use Expr instead. Use `ExprKind::Let` to represent if let guards.

compiler/rustc_hir_analysis/src/check/region.rs

@@ -184,7 +184,8 @@ fn resolve_arm<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, arm: &'tcx hir
     visitor.enter_node_scope_with_dtor(arm.hir_id.local_id);
     visitor.cx.var_parent = visitor.cx.parent;
 
-    if let Some(hir::Guard::If(expr)) = arm.guard {
+    if let Some(expr) = arm.guard {
+        // Check for if??
         visitor.terminating_scopes.insert(expr.hir_id.local_id);
     }