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/AMDGPU/SIPeepholeSDWA.cpp b/llvm/lib/Target/AMDGPU/SIPeepholeSDWA.cpp
index 7b4652e..5ed7fdf 100644
--- a/llvm/lib/Target/AMDGPU/SIPeepholeSDWA.cpp
+++ b/llvm/lib/Target/AMDGPU/SIPeepholeSDWA.cpp
@@ -1050,7 +1050,7 @@
 bool SIPeepholeSDWA::runOnMachineFunction(MachineFunction &MF) {
   const SISubtarget &ST = MF.getSubtarget<SISubtarget>();
 
-  if (!ST.hasSDWA() || skipFunction(*MF.getFunction()))
+  if (!ST.hasSDWA() || skipFunction(MF.getFunction()))
     return false;
 
   MRI = &MF.getRegInfo();