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/SIInsertSkips.cpp b/llvm/lib/Target/AMDGPU/SIInsertSkips.cpp
index 1b8c9f2..a2f844d 100644
--- a/llvm/lib/Target/AMDGPU/SIInsertSkips.cpp
+++ b/llvm/lib/Target/AMDGPU/SIInsertSkips.cpp
@@ -166,7 +166,7 @@
   MachineBasicBlock &MBB = *MI.getParent();
   MachineFunction *MF = MBB.getParent();
 
-  if (MF->getFunction()->getCallingConv() != CallingConv::AMDGPU_PS ||
+  if (MF->getFunction().getCallingConv() != CallingConv::AMDGPU_PS ||
       !shouldSkip(MBB, MBB.getParent()->back()))
     return false;