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/HexagonSplitDouble.cpp b/llvm/lib/Target/Hexagon/HexagonSplitDouble.cpp
index 68b5ddd..c9f5400 100644
--- a/llvm/lib/Target/Hexagon/HexagonSplitDouble.cpp
+++ b/llvm/lib/Target/Hexagon/HexagonSplitDouble.cpp
@@ -1163,7 +1163,7 @@
   DEBUG(dbgs() << "Splitting double registers in function: "
         << MF.getName() << '\n');
 
-  if (skipFunction(*MF.getFunction()))
+  if (skipFunction(MF.getFunction()))
     return false;
 
   auto &ST = MF.getSubtarget<HexagonSubtarget>();