Enable HASH_CODE by default to avoid repeated hash computation

On some SV-COMP benchmark categories, hashing accounts for >20% of CPU
time (with profiling enabled) - top five:

* ReachSafety-BitVectors: 29.29% (470.54 seconds, which reduces to 4.39
  seconds; for benchmarks not timing out we save 170 seconds (25%) in
  non-profiling mode)
* Systems_BusyBox_NoOverflows: 27.98% (284.15 seconds, which reduces to
  1.74 seconds; for the 1 benchmark not timing out we save 23 seconds
  (6%) in non-profiling mode)
* Systems_BusyBox_MemSafety: 24.24% (194.74 seconds, which reduces to
  0.93 seconds; no measurable difference on the 2 benchmarks not
  failing/timing out)
* NoOverflows-Other: 18.84% (1127.61 seconds, which reduces to
  23.57 seconds; for benchmarks not timing out we save 5 seconds (7%) in
  non-profiling mode)
* ReachSafety-ControlFlow: 17.75% (1194.04 seconds, which reduces to
  29.17 seconds; for benchmarks not timing out we save 200 seconds (25%)
  in non-profiling mode)

For ReachSafety-ECA it's only 4.7%, which still amounts to 3006.7
seconds. With this change this reduces to 323.07 seconds. On
ReachSafety-ECA, this enables 3055.35 symex_step calls per second over
2752.28 calls per second without this change.

Author: tautschnig

src/util/irep.h

@@ -17,7 +17,9 @@ Author: Daniel Kroening, [email protected]
 #include "irep_ids.h"
 
 #define SHARING
-// #define HASH_CODE
+#ifndef HASH_CODE
+#  define HASH_CODE
+#endif
 // #define NAMED_SUB_IS_FORWARD_LIST
 
 #ifdef NAMED_SUB_IS_FORWARD_LIST