[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