[llvm-exegesis][NFC] Disassociate snippet generators from benchmark runners

The addition of `inverse_throughput` mode highlighted the disjointedness
of snippet generators and benchmark runners because it used the
`UopsSnippetGenerator` with the  `LatencyBenchmarkRunner`.
To keep the code consistent tie the snippet generators to
parallelization/serialization rather than their benchmark runners.

Renaming `LatencySnippetGenerator` -> `SerialSnippetGenerator`.
Renaming `UopsSnippetGenerator` -> `ParallelSnippetGenerator`.

Differential Revision: https://reviews.llvm.org/D72928
diff --git a/llvm/tools/llvm-exegesis/lib/Target.h b/llvm/tools/llvm-exegesis/lib/Target.h
index 277db67..62f0dab 100644
--- a/llvm/tools/llvm-exegesis/lib/Target.h
+++ b/llvm/tools/llvm-exegesis/lib/Target.h
@@ -144,9 +144,9 @@
 
   // Targets can implement their own snippet generators/benchmarks runners by
   // implementing these.
-  std::unique_ptr<SnippetGenerator> virtual createLatencySnippetGenerator(
+  std::unique_ptr<SnippetGenerator> virtual createSerialSnippetGenerator(
       const LLVMState &State, const SnippetGenerator::Options &Opts) const;
-  std::unique_ptr<SnippetGenerator> virtual createUopsSnippetGenerator(
+  std::unique_ptr<SnippetGenerator> virtual createParallelSnippetGenerator(
       const LLVMState &State, const SnippetGenerator::Options &Opts) const;
   std::unique_ptr<BenchmarkRunner> virtual createLatencyBenchmarkRunner(
       const LLVMState &State, InstructionBenchmark::ModeE Mode) const;