[llvm-exegesis] Use BenchmarkResult::Instructions instead of OpcodeName

Summary:
Get rid of OpcodeName.

To remove the opcode name from an old file:
```
cat old_file | sed '/opcode_name.*/d'
```

Reviewers: gchatelet

Subscribers: tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D48121

llvm-svn: 334691
diff --git a/llvm/tools/llvm-exegesis/lib/BenchmarkResult.cpp b/llvm/tools/llvm-exegesis/lib/BenchmarkResult.cpp
index 237a540..841219c 100644
--- a/llvm/tools/llvm-exegesis/lib/BenchmarkResult.cpp
+++ b/llvm/tools/llvm-exegesis/lib/BenchmarkResult.cpp
@@ -140,8 +140,7 @@
 
 template <> struct MappingTraits<exegesis::InstructionBenchmarkKey> {
   static void mapping(IO &Io, exegesis::InstructionBenchmarkKey &Obj) {
-    Io.mapRequired("opcode_name", Obj.OpcodeName);
-    Io.mapOptional("instructions", Obj.Instructions);
+    Io.mapRequired("instructions", Obj.Instructions);
     Io.mapOptional("config", Obj.Config);
   }
 };