[llvm-exegesis][NFC] Split BenchmarkRunner class

Summary:
The snippet-generation part goes to the SnippetGenerator class.

This will allow benchmarking arbitrary code (see PR38437).

Reviewers: gchatelet

Subscribers: mgorny, tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D51979

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@342117 91177308-0d34-0410-b5e6-96231b3b80d8
15 files changed