Restore conditional bounds-checking of count_{leading,trailing}_zeros

The cleanup of 4d4f9e7 (PR #6684) made bounds checking of these bit
count expressions unconditional. This did not affect any input coming
from the C front-end, but became apparent with the Rust front-end as
documented in model-checking/kani#886.

This commit restores conditional bounds checking, but now actually uses
the proper expression API rather than the low-level hack.

Author: tautschnig

src/ansi-c/goto_check_c.cpp

@@ -1508,7 +1508,10 @@ void goto_check_ct::bounds_check(const exprt &expr, const guardt &guard)
   if(expr.id() == ID_index)
     bounds_check_index(to_index_expr(expr), guard);
   else if(
-    expr.id() == ID_count_leading_zeros || expr.id() == ID_count_trailing_zeros)
+    (expr.id() == ID_count_leading_zeros &&
+     !to_count_leading_zeros_expr(expr).zero_permitted()) ||
+    (expr.id() == ID_count_trailing_zeros &&
+     !to_count_trailing_zeros_expr(expr).zero_permitted()))
   {
     bounds_check_bit_count(to_unary_expr(expr), guard);
   }