Fix computation of next temporary in -canonical-idents mode

Variables were confused for temporaries, causing the temporaries introduced
by this pass to be very big integers.

Fixes: #370

Author: xavierleroy

exportclight/Clightnorm.ml

@@ -143,7 +143,18 @@ and norm_lbl_stmt ls =
   | LSnil -> LSnil
   | LScons(n, s, ls) -> LScons(n, norm_stmt s, norm_lbl_stmt ls)
 
-let next_var curr (v, _) = if P.lt v curr then curr else P.succ v
+(* In "canonical atoms" mode, temporaries are between 2^7 and 2^12 - 1.
+   Below 2^7 are single-letter identifiers and above 2^12 are all
+   other identifiers. *)
+
+let first_temp = P.of_int 128
+let last_temp  = P.of_int 4095
+
+let next_var curr (v, _) =
+  if P.lt v curr
+  || !use_canonical_atoms && (P.lt v first_temp || P.gt v last_temp)
+  then curr
+  else P.succ v
 
 let next_var_list vars start = List.fold_left next_var start vars