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/GCNIterativeScheduler.cpp b/llvm/lib/Target/AMDGPU/GCNIterativeScheduler.cpp
index 178d993..a0e4f7f 100644
--- a/llvm/lib/Target/AMDGPU/GCNIterativeScheduler.cpp
+++ b/llvm/lib/Target/AMDGPU/GCNIterativeScheduler.cpp
@@ -566,7 +566,7 @@
   bool TryMaximizeOccupancy) {
   const auto &ST = MF.getSubtarget<SISubtarget>();
   auto TgtOcc = std::min(ST.getOccupancyWithLocalMemSize(MF),
-                         ST.getWavesPerEU(*MF.getFunction()).second);
+                         ST.getWavesPerEU(MF.getFunction()).second);
 
   sortRegionsByPressure(TgtOcc);
   auto Occ = Regions.front()->MaxPressure.getOccupancy(ST);