[MachineCombiner] Don't use the opcode-only form of computeInstrLatency

In r242277, I updated the MachineCombiner to work with itineraries, but I
missed a call that is scheduling-model-only (the opcode-only form of
computeInstrLatency). Using the form that takes an MI* allows this to work with
itineraries (and should be NFC for subtargets with scheduling models).

llvm-svn: 244020
diff --git a/llvm/lib/CodeGen/MachineCombiner.cpp b/llvm/lib/CodeGen/MachineCombiner.cpp
index b5b5ac0..aab436f 100644
--- a/llvm/lib/CodeGen/MachineCombiner.cpp
+++ b/llvm/lib/CodeGen/MachineCombiner.cpp
@@ -204,7 +204,7 @@
           NewRoot, NewRoot->findRegisterDefOperandIdx(MO.getReg()), UseMO,
           UseMO->findRegisterUseOperandIdx(MO.getReg()));
     } else {
-      LatencyOp = TSchedModel.computeInstrLatency(NewRoot->getOpcode());
+      LatencyOp = TSchedModel.computeInstrLatency(NewRoot);
     }
     NewRootLatency = std::max(NewRootLatency, LatencyOp);
   }