[llvm-exegesis] Allow benchmarking arbitrary code snippets.

Summary:

This is a step towards fixing PR38048.

Note that right now the measurements are given per instruction. We'll
need to give measurements a per code snippet and update the analysis (PR38731).

Reviewers: gchatelet

Subscribers: tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D52041

llvm-svn: 342947