Unrevert #1131 and fix a Cmm unboxing bug (#1284)

* Revert "Revert "Make Selectgen treat region boundaries more precisely" (#1283)"

This reverts commit abd91a9.

* Fix bad interaction between Cmm unboxing and locals

Local regions should not count as "tail" for Cmm unboxing purposes.

Author: stedolan

backend/cmm.ml

@@ -319,14 +319,10 @@ let iter_shallow_tail f = function
       f e1;
       f e2;
       true
-  | Cregion e ->
-      f e;
-      true
-  | Ctail e ->
-      f e;
-      true
   | Cexit _ | Cop (Craise _, _, _) ->
       true
+  | Cregion _
+  | Ctail _
   | Cconst_int _
   | Cconst_natint _
   | Cconst_float _
@@ -365,12 +361,10 @@ let map_shallow_tail ?kind f = function
   | Ctrywith(e1, kind', id, e2, dbg, kind_before) ->
       Ctrywith(f e1, kind', id, f e2, dbg,
               Option.value kind ~default:kind_before)
-  | Cregion e ->
-      Cregion(f e)
-  | Ctail e ->
-      Ctail(f e)
   | Cexit _ | Cop (Craise _, _, _) as cmm ->
       cmm
+  | Cregion _
+  | Ctail _
   | Cconst_int _
   | Cconst_natint _
   | Cconst_float _
@@ -382,6 +376,8 @@ let map_shallow_tail ?kind f = function
 
 let map_tail ?kind f =
   let rec loop = function
+    | Cregion _
+    | Ctail _
     | Cconst_int _
     | Cconst_natint _
     | Cconst_float _

backend/cmmgen.ml

@@ -394,7 +394,8 @@ let rec is_unboxed_number_cmm = function
           No_unboxing
       end
     | Cexit _ | Cop (Craise _, _, _) -> No_result
-    | Csequence (_, a) | Cregion a | Ctail a
+    | Cregion _ | Ctail _ -> No_unboxing
+    | Csequence (_, a)
     | Clet (_, _, a) | Cphantom_let (_, _, a) | Clet_mut (_, _, _, a) ->
       is_unboxed_number_cmm a
     | Cconst_int _