[llvm-exegesis][NFC] Simplify code.

Summary:
What we're redoing already exists in the X86 backend, it's called
`X86II::getOperandBias`.

Reviewers: gchatelet

Subscribers: tschuett, mstojanovic, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73340