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/SystemZShortenInst.cpp b/llvm/lib/Target/SystemZ/SystemZShortenInst.cpp
index d9c8fab..195fa20 100644
--- a/llvm/lib/Target/SystemZ/SystemZShortenInst.cpp
+++ b/llvm/lib/Target/SystemZ/SystemZShortenInst.cpp
@@ -309,7 +309,7 @@
 }
 
 bool SystemZShortenInst::runOnMachineFunction(MachineFunction &F) {
-  if (skipFunction(*F.getFunction()))
+  if (skipFunction(F.getFunction()))
     return false;
 
   const SystemZSubtarget &ST = F.getSubtarget<SystemZSubtarget>();