Closure conversion: Filter variables out of subst under lambda (#514)

PR #470 propagates substitutions down into lambdas, which works when the
RHS is a constant or symbol but not if it's a variable, since that
variable is now out of scope. This was breaking #485, which produces
variable-for-variable substitutions often. Fortunately,
variable-for-variable substitution under a lambda is also unnecessary
(we already dealt with the free occurrences by making a closure element),
so we can happily just filter out any such bindings.

middle_end/flambda/from_lambda/closure_conversion_aux.ml

@@ -112,6 +112,10 @@ module Env = struct
   let clear_local_bindings
         { variables = _; globals; simples_to_substitute; backend;
           current_unit_id; symbol_for_global'; } =
+    let simples_to_substitute =
+      Ident.Map.filter (fun _ simple -> not (Simple.is_var simple))
+        simples_to_substitute
+    in
     { variables = Ident.Map.empty;
       globals;
       simples_to_substitute;