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/ARM/MLxExpansionPass.cpp b/llvm/lib/Target/ARM/MLxExpansionPass.cpp
index 00c41c4..153e7b1 100644
--- a/llvm/lib/Target/ARM/MLxExpansionPass.cpp
+++ b/llvm/lib/Target/ARM/MLxExpansionPass.cpp
@@ -371,7 +371,7 @@
 }
 
 bool MLxExpansion::runOnMachineFunction(MachineFunction &Fn) {
-  if (skipFunction(*Fn.getFunction()))
+  if (skipFunction(Fn.getFunction()))
     return false;
 
   TII = static_cast<const ARMBaseInstrInfo *>(Fn.getSubtarget().getInstrInfo());