[llvm-mca] Fix unused variable warning in opt mode.

llvm-svn: 327394
diff --git a/llvm/tools/llvm-mca/Backend.h b/llvm/tools/llvm-mca/Backend.h
index c2532d8..c01d015 100644
--- a/llvm/tools/llvm-mca/Backend.h
+++ b/llvm/tools/llvm-mca/Backend.h
@@ -86,7 +86,7 @@
     const auto It = Instructions.find(Index);
     assert(It != Instructions.end() && "no running instructions with index");
     assert(It->second);
-    return *Instructions.find(Index)->second;
+    return *It->second;
   }
   void eraseInstruction(unsigned Index) { Instructions.erase(Index); }
   unsigned getNumCycles() const { return Cycles; }