ipa-cp: One more safe_add (PR 97816)

The new behavior of safe_add triggered an ICE because of one use where
it had not been used instead of a simple addition.  I'll fix it with the
following obvious patch so that periodic benchmarkers can continue
working because a proper fix (see below) will need a review.

The testcase showed me, however, that we can propagate time and cost
from one lattice to another more than once even when that was not the
intent.  I'll address that as a follow-up after I verify it does not
affect the IPA-CP heuristics too much or change the corresponding
params accordingly.

Bootstrapped and tested on x86_64-linux.

gcc/ChangeLog:

2020-11-13  Martin Jambor  <mjambor@suse.cz>

	PR ipa/97816
	* ipa-cp.c (value_topo_info<valtype>::propagate_effects): Use
	safe_add instead of a simple addition.

Author: jamborm

gcc/ipa-cp.c

@@ -3873,7 +3873,8 @@ value_topo_info<valtype>::propagate_effects ()
       for (val = base; val; val = val->scc_next)
 	{
 	  time = time + val->local_time_benefit + val->prop_time_benefit;
-	  size = safe_add (size, val->local_size_cost + val->prop_size_cost);
+	  size = safe_add (size, safe_add (val->local_size_cost,
+					   val->prop_size_cost));
 	}
 
       for (val = base; val; val = val->scc_next)