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/MachineSink.cpp b/llvm/lib/CodeGen/MachineSink.cpp
index fd4bd1a..bedfdd8 100644
--- a/llvm/lib/CodeGen/MachineSink.cpp
+++ b/llvm/lib/CodeGen/MachineSink.cpp
@@ -292,7 +292,7 @@
 }
 
 bool MachineSinking::runOnMachineFunction(MachineFunction &MF) {
-  if (skipFunction(*MF.getFunction()))
+  if (skipFunction(MF.getFunction()))
     return false;
 
   DEBUG(dbgs() << "******** Machine Sinking ********\n");