[llvm-exegesis] Remove unused variable, add more semantic to Instruction.

Reviewers: courbet

Subscribers: tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D53062

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@344127 91177308-0d34-0410-b5e6-96231b3b80d8