[llvm-exegesis] Use LLVMTargetMachine pointer everywhere. NFCI.

Avoid calling the unique_ptr multiple times.

llvm-svn: 330260
diff --git a/llvm/tools/llvm-exegesis/lib/InMemoryAssembler.cpp b/llvm/tools/llvm-exegesis/lib/InMemoryAssembler.cpp
index f110d00..2456807 100644
--- a/llvm/tools/llvm-exegesis/lib/InMemoryAssembler.cpp
+++ b/llvm/tools/llvm-exegesis/lib/InMemoryAssembler.cpp
@@ -206,7 +206,7 @@
   ExecEngine.reset(
       llvm::EngineBuilder(std::move(FunctionContext.Module))
           .setErrorStr(&Error)
-          .setMCPU(FunctionContext.TM->getTargetCPU())
+          .setMCPU(TM->getTargetCPU())
           .setEngineKind(llvm::EngineKind::JIT)
           .setMCJITMemoryManager(
               llvm::make_unique<TrackingSectionMemoryManager>(&CodeSize))