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/XRayInstrumentation.cpp b/llvm/lib/CodeGen/XRayInstrumentation.cpp
index 60ac24e..3d83afc 100644
--- a/llvm/lib/CodeGen/XRayInstrumentation.cpp
+++ b/llvm/lib/CodeGen/XRayInstrumentation.cpp
@@ -142,7 +142,7 @@
 }
 
 bool XRayInstrumentation::runOnMachineFunction(MachineFunction &MF) {
-  auto &F = *MF.getFunction();
+  auto &F = MF.getFunction();
   auto InstrAttr = F.getFnAttribute("function-instrument");
   bool AlwaysInstrument = !InstrAttr.hasAttribute(Attribute::None) &&
                           InstrAttr.isStringAttribute() &&