Don't accidentally create MachineFunctions in mir-debugify/mir-strip-debugify

We should only modify existing ones. Previously, we were creating
MachineFunctions for externally-available functions. AFAICT this was benign
in tree but ultimately led to asan bugs in our out of tree target.

Author: dsandersllvm

llvm/lib/CodeGen/MachineDebugify.cpp

@@ -27,7 +27,10 @@ using namespace llvm;
 namespace {
 bool applyDebugifyMetadataToMachineFunction(MachineModuleInfo &MMI,
                                             DIBuilder &DIB, Function &F) {
-  MachineFunction &MF = MMI.getOrCreateMachineFunction(F);
+  MachineFunction *MaybeMF = MMI.getMachineFunction(F);
+  if (!MaybeMF)
+    return false;
+  MachineFunction &MF = *MaybeMF;
 
   DISubprogram *SP = F.getSubprogram();
   assert(SP && "IR Debugify just created it?");

llvm/lib/CodeGen/MachineStripDebug.cpp

@@ -45,7 +45,10 @@ struct StripDebugMachineModule : public ModulePass {
 
     bool Changed = false;
     for (Function &F : M.functions()) {
-      MachineFunction &MF = MMI.getOrCreateMachineFunction(F);
+      MachineFunction *MaybeMF = MMI.getMachineFunction(F);
+      if (!MaybeMF)
+        continue;
+      MachineFunction &MF = *MaybeMF;
       for (MachineBasicBlock &MBB : MF) {
         for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end();
              I != E;) {