[llvm-exegesis] Serial snippet: Restrict the set of back-to-back instructions

Summary:
Right now when picking a back-to-back instruction at random, we might select
instructions that we do not know how to handle.
Add a ExegesisTarget hook to possibly filter instructions.

Reviewers: gchatelet

Subscribers: tschuett, mstojanovic, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73161