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/PostRASchedulerList.cpp b/llvm/lib/CodeGen/PostRASchedulerList.cpp
index a03bb68..5d86faa 100644
--- a/llvm/lib/CodeGen/PostRASchedulerList.cpp
+++ b/llvm/lib/CodeGen/PostRASchedulerList.cpp
@@ -279,7 +279,7 @@
 }
 
 bool PostRAScheduler::runOnMachineFunction(MachineFunction &Fn) {
-  if (skipFunction(*Fn.getFunction()))
+  if (skipFunction(Fn.getFunction()))
     return false;
 
   TII = Fn.getSubtarget().getInstrInfo();