[llvm-exegesis][NFC] Use accessors for Operand.

Summary:
This moves checking logic into the accessors and makes the structure smaller.
It will also help when/if Operand are generated from the TD files.

Subscribers: tschuett, courbet, llvm-commits

Differential Revision: https://reviews.llvm.org/D52982

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@344028 91177308-0d34-0410-b5e6-96231b3b80d8