[libFuzzer] add -trace_cmp=1 (guiding mutations based on the observed CMP instructions). This is a reincarnation of the previously deleted -use_traces, but using a different approach for collecting traces. Still a toy, but at least it scales well. Also fix -merge in trace-pc-guard mode

llvm-svn: 284273
diff --git a/llvm/lib/Fuzzer/FuzzerOptions.h b/llvm/lib/Fuzzer/FuzzerOptions.h
index d0bac9c..59126a2 100644
--- a/llvm/lib/Fuzzer/FuzzerOptions.h
+++ b/llvm/lib/Fuzzer/FuzzerOptions.h
@@ -30,6 +30,7 @@
   bool UseIndirCalls = true;
   bool UseMemcmp = true;
   bool UseMemmem = true;
+  bool UseCmp = false;
   bool UseValueProfile = false;
   bool Shrink = false;
   int ReloadIntervalSec = 1;