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/CodeGen/GlobalISel/Legalizer.cpp b/llvm/lib/CodeGen/GlobalISel/Legalizer.cpp
index d026aca..f09b0d9 100644
--- a/llvm/lib/CodeGen/GlobalISel/Legalizer.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/Legalizer.cpp
@@ -175,7 +175,7 @@
   // outerloop for that.
   if (MF.size() != NumBlocks) {
     MachineOptimizationRemarkMissed R("gisel-legalize", "GISelFailure",
-                                      MF.getFunction()->getSubprogram(),
+                                      MF.getFunction().getSubprogram(),
                                       /*MBB=*/nullptr);
     R << "inserting blocks is not supported yet";
     reportGISelFailure(MF, TPC, MORE, R);