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/Target/SystemZ/SystemZElimCompare.cpp b/llvm/lib/Target/SystemZ/SystemZElimCompare.cpp
index ca82740..55f7a7b 100644
--- a/llvm/lib/Target/SystemZ/SystemZElimCompare.cpp
+++ b/llvm/lib/Target/SystemZ/SystemZElimCompare.cpp
@@ -593,7 +593,7 @@
 }
 
 bool SystemZElimCompare::runOnMachineFunction(MachineFunction &F) {
-  if (skipFunction(*F.getFunction()))
+  if (skipFunction(F.getFunction()))
     return false;
 
   TII = static_cast<const SystemZInstrInfo *>(F.getSubtarget().getInstrInfo());