ipa-utils: avoid uninitialized probabilities on ICF [PR111559]

r14-3459-g0c78240fd7d519 "Check that passes do not forget to define profile"
exposed check failures in cases when gcc produces uninitialized profile
probabilities. In case of PR/111559 uninitialized profile is generated
by edges executed 0 times reported by IPA profile:

    $ gcc -O2 -fprofile-generate pr111559.c -o b -fopt-info
    $ ./b
    $ gcc -O2 -fprofile-use -fprofile-correction pr111559.c -o b -fopt-info

    pr111559.c: In function 'rule1':
    pr111559.c:6:13: error: probability of edge 3->4 not initialized
        6 | static void rule1(void) { if (p) edge(); }
          |             ^~~~~
    during GIMPLE pass: fixup_cfg
    pr111559.c:6:13: internal compiler error: verify_flow_info failed

The change conservatively ignores updates with zero execution counts and
uses initially assigned probabilities (`always` probability in case of
the example).

	PR ipa/111283
	PR gcov-profile/111559

gcc/
	* ipa-utils.cc (ipa_merge_profiles): Avoid producing
	uninitialized probabilities when merging counters with zero
	denominators.

gcc/testsuite/
	* gcc.dg/tree-prof/pr111559.c: New test.

gcc/ipa-utils.cc

@@ -651,13 +651,14 @@ ipa_merge_profiles (struct cgraph_node *dst,
 		{
 		  edge srce = EDGE_SUCC (srcbb, i);
 		  edge dste = EDGE_SUCC (dstbb, i);
-		  dste->probability = 
-		    dste->probability * dstbb->count.ipa ().probability_in
-						 (dstbb->count.ipa ()
-						  + srccount.ipa ())
-		    + srce->probability * srcbb->count.ipa ().probability_in
-						 (dstbb->count.ipa ()
-						  + srccount.ipa ());
+		  profile_count sum =
+		    dstbb->count.ipa () + srccount.ipa ();
+		  if (sum.nonzero_p ())
+		    dste->probability =
+		      dste->probability * dstbb->count.ipa ().probability_in
+						   (sum)
+		      + srce->probability * srcbb->count.ipa ().probability_in
+						   (sum);
 		}
 	      dstbb->count = dstbb->count.ipa () + srccount.ipa ();
 	    }

gcc/testsuite/gcc.dg/tree-prof/pr111559.c

@@ -0,0 +1,16 @@
+/* { dg-options "-O2" } */
+
+__attribute__((noipa)) static void edge(void) {}
+
+int p = 0;
+
+__attribute__((noinline))
+static void rule1(void) { if (p) edge(); }
+
+__attribute__((noinline))
+static void rule1_same(void) { if (p) edge(); }
+
+__attribute__((noipa)) int main(void) {
+    rule1();
+    rule1_same();
+}