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/GlobalISel/CallLowering.cpp b/llvm/lib/CodeGen/GlobalISel/CallLowering.cpp
index 50ea69a..114c068 100644
--- a/llvm/lib/CodeGen/GlobalISel/CallLowering.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/CallLowering.cpp
@@ -108,7 +108,7 @@
                                      ArrayRef<ArgInfo> Args,
                                      ValueHandler &Handler) const {
   MachineFunction &MF = MIRBuilder.getMF();
-  const Function &F = *MF.getFunction();
+  const Function &F = MF.getFunction();
   const DataLayout &DL = F.getParent()->getDataLayout();
 
   SmallVector<CCValAssign, 16> ArgLocs;