MachineFunction: Return reference from getFunction(); NFC

The Function can never be nullptr so we can return a reference.

llvm-svn: 320884
diff --git a/llvm/lib/CodeGen/MachineLICM.cpp b/llvm/lib/CodeGen/MachineLICM.cpp
index a251a08..75d449c 100644
--- a/llvm/lib/CodeGen/MachineLICM.cpp
+++ b/llvm/lib/CodeGen/MachineLICM.cpp
@@ -280,7 +280,7 @@
 }
 
 bool MachineLICM::runOnMachineFunction(MachineFunction &MF) {
-  if (skipFunction(*MF.getFunction()))
+  if (skipFunction(MF.getFunction()))
     return false;
 
   Changed = FirstInLoop = false;