[llvm-exegesis] Get the BenchmarkRunner from the ExegesisTarget.

Summary:
This allows targets to override code generation for some instructions.
As an example of override, this also moves ad-hoc instruction filtering
for X86 into the X86 ExegesisTarget.

Reviewers: gchatelet

Subscribers: mgorny, tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D48587

llvm-svn: 335582
diff --git a/llvm/unittests/tools/llvm-exegesis/X86/SnippetGeneratorTest.cpp b/llvm/unittests/tools/llvm-exegesis/X86/SnippetGeneratorTest.cpp
index 77de5e7..edacd7f 100644
--- a/llvm/unittests/tools/llvm-exegesis/X86/SnippetGeneratorTest.cpp
+++ b/llvm/unittests/tools/llvm-exegesis/X86/SnippetGeneratorTest.cpp
@@ -217,17 +217,14 @@
 
 class FakeBenchmarkRunner : public BenchmarkRunner {
 public:
-  using BenchmarkRunner::BenchmarkRunner;
+  FakeBenchmarkRunner(const LLVMState &State)
+      : BenchmarkRunner(State, InstructionBenchmark::Unknown) {}
 
   Instruction createInstruction(unsigned Opcode) {
     return Instruction(State.getInstrInfo().get(Opcode), RATC);
   }
 
 private:
-  InstructionBenchmark::ModeE getMode() const override {
-    return InstructionBenchmark::Unknown;
-  }
-
   llvm::Expected<SnippetPrototype>
   generatePrototype(unsigned Opcode) const override {
     return llvm::make_error<llvm::StringError>("not implemented",