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/PowerPC/PPCVSXFMAMutate.cpp b/llvm/lib/Target/PowerPC/PPCVSXFMAMutate.cpp
index 04fa358..f15af79 100644
--- a/llvm/lib/Target/PowerPC/PPCVSXFMAMutate.cpp
+++ b/llvm/lib/Target/PowerPC/PPCVSXFMAMutate.cpp
@@ -343,7 +343,7 @@
 
 public:
     bool runOnMachineFunction(MachineFunction &MF) override {
-      if (skipFunction(*MF.getFunction()))
+      if (skipFunction(MF.getFunction()))
         return false;
 
       // If we don't have VSX then go ahead and return without doing