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/Hexagon/HexagonRDFOpt.cpp b/llvm/lib/Target/Hexagon/HexagonRDFOpt.cpp
index c73a230..413bc8e 100644
--- a/llvm/lib/Target/Hexagon/HexagonRDFOpt.cpp
+++ b/llvm/lib/Target/Hexagon/HexagonRDFOpt.cpp
@@ -280,7 +280,7 @@
 }
 
 bool HexagonRDFOpt::runOnMachineFunction(MachineFunction &MF) {
-  if (skipFunction(*MF.getFunction()))
+  if (skipFunction(MF.getFunction()))
     return false;
 
   if (RDFLimit.getPosition()) {