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/PPCMIPeephole.cpp b/llvm/lib/Target/PowerPC/PPCMIPeephole.cpp
index 64c5e4e..27ded63 100644
--- a/llvm/lib/Target/PowerPC/PPCMIPeephole.cpp
+++ b/llvm/lib/Target/PowerPC/PPCMIPeephole.cpp
@@ -106,7 +106,7 @@
 
   // Main entry point for this pass.
   bool runOnMachineFunction(MachineFunction &MF) override {
-    if (skipFunction(*MF.getFunction()))
+    if (skipFunction(MF.getFunction()))
       return false;
     initialize(MF);
     return simplifyCode();