[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

llvm-svn: 344028