[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

llvm-svn: 342117
diff --git a/llvm/tools/llvm-exegesis/lib/CMakeLists.txt b/llvm/tools/llvm-exegesis/lib/CMakeLists.txt
index 175c2ad..17e6a98 100644
--- a/llvm/tools/llvm-exegesis/lib/CMakeLists.txt
+++ b/llvm/tools/llvm-exegesis/lib/CMakeLists.txt
@@ -19,6 +19,7 @@
   MCInstrDescView.cpp
   PerfHelper.cpp
   RegisterAliasing.cpp
+  SnippetGenerator.cpp
   Target.cpp
   Uops.cpp
   )