Improve error handling: replace assert false with ICE and fix SList dead code elimination

Two improvements to internal error handling and optimization consistency:

1. Replace bare `assert false` in Info.ml with a proper
   ICE.internal_compiler_error call that includes the offending type in
   the error message. This produces a helpful diagnostic with a link to
   the issue tracker instead of a generic assertion failure if
   this unreachable branch is ever hit.

2. Fix inconsistent SList handling in dead_code_elim: Block and Profile
   both collapse to Skip when all children are eliminated, but SList
   was left as an empty SList []. This is inconsistent and produces
   unnecessary nodes that Mir_utils.cleanup_stmts already treats as
   equivalent to Skip (line 435).

Author: ishaan-arora-1

src/analysis_and_optimization/Optimize.ml

@@ -805,7 +805,7 @@ let dead_code_elimination (mir : Program.Typed.t) =
           if List.length l' = 0 then Skip else Block l'
       | SList l ->
           let l' = List.filter ~f:(fun x -> x.pattern <> Skip) l in
-          SList l' in
+          if List.is_empty l' then Skip else SList l' in
     let dead_code_elim_stmt =
       map_rec_stmt_loc_num flowgraph_to_mir dead_code_elim_stmt_base in
     dead_code_elim_stmt (Map.find_exn flowgraph_to_mir 1) in

src/frontend/Info.ml

@@ -19,7 +19,10 @@ let rec unsized_basetype_json t =
       `Assoc
         [ ("type", `List (List.map ~f:unsized_basetype_json internals))
         ; ("dimensions", `Int dims) ]
-  | UMathLibraryFunction | UFun _ | UArray _ -> assert false
+  | (UMathLibraryFunction | UFun _ | UArray _) as t ->
+      Common.ICE.internal_compiler_error
+        [%message
+          "Unexpected unsized type in unsized_basetype_json" (t : UnsizedType.t)]
 
 let basetype_dims t = SizedType.to_unsized t |> unsized_basetype_json