[NewPM] Fix -print-changed when a -filter-print-funcs function is removed

[aeubanks]

Summary

-filter-print-funcs -print-changed was crashing after the filter func
was removed by a pass with

Assertion failed: After.find("*** IR Dump") == 0 && "Unexpected banner format."

We weren't printing the banner because when we have -filter-print-funcs,
we print each function separately, letting the print function filter out
unwanted functions.

[jamieschmeiser]

Do you think that the IR being deleted should be reported rather than silently disappearing from the trace? Perhaps with

Out << "*** IR Deleted" << Name << " ***\n"; 
```before the return?

[aeubanks]

print "IR Deleted"

[jamieschmeiser]

LGTM

llvm/lib/Passes/StandardInstrumentations.cpp

unwrapAndPrint(OS, IR, Banner, forcePrintModuleIR(),
                  /*Brief=*/false, /*ShouldPreserveUseListOrder=*/true);
 
   OS.str();
 }
 
 void IRChangedPrinter::handleAfter(StringRef PassID, std::string &Name,
                                    const std::string &Before,
                                    const std::string &After, Any) {
+  // We might not get anything to print if we only want to print a specific
+  // function but it gets deleted.
+  if (After.empty()) {
+    Out << "*** IR Deleted After " << PassID << Name << " ***\n";
+    return;
+  }
+
   assert(After.find("*** IR Dump") == 0 && "Unexpected banner format.");
   StringRef AfterRef = After;
   StringRef Banner =
       AfterRef.take_until([](char C) -> bool { return C == '\n'; });
 
   // Report the IR before the changes when requested.
   if (PrintChangedBefore) {
     Out << "*** IR Dump Before" << Banner.substr(17);

llvm/test/Other/print-changed-deleted.ll

+; RUN: opt -passes=globaldce < %s -disable-output -print-changed -filter-print-funcs=f 2>&1 | FileCheck %s
+
+; CHECK-NOT: IR Dump After GlobalDCEPass
+; CHECK: IR Deleted After GlobalDCEPass
+; CHECK-NOT: IR Dump After GlobalDCEPass
+
+declare void @f()