Fix (optimizer) crashes in conditional nodes when result is not needed.

tests/language/nullaware_opt_test.dart

Index: tests/language/nullaware_opt_test.dart
diff --git a/tests/language/nullaware_opt_test.dart b/tests/language/nullaware_opt_test.dart
index 096c163e47cf5fc0f1c43e646f0649918aece35b..aafb7ac5a149e0899978e04f5cbad6921e695d15 100644
--- a/tests/language/nullaware_opt_test.dart
+++ b/tests/language/nullaware_opt_test.dart
@@ -57,8 +57,16 @@ test() {
   var x = 5 ?? bomb();
 }
 
+// Check that instructions without result do not crash.
+test2() {
+  var c;
+  c?.v;
+  c?.m(bomb());
+}
+
 main() {
   for (int i = 0; i < 10; i++) {
     test();
+    test2();
   }
 }