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/AArch64/AArch64LoadStoreOptimizer.cpp b/llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp
index c406228..8a29456 100644
--- a/llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp
+++ b/llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp
@@ -1759,7 +1759,7 @@
 }
 
 bool AArch64LoadStoreOpt::runOnMachineFunction(MachineFunction &Fn) {
-  if (skipFunction(*Fn.getFunction()))
+  if (skipFunction(Fn.getFunction()))
     return false;
 
   Subtarget = &static_cast<const AArch64Subtarget &>(Fn.getSubtarget());