Fix bad assert in print-changed code

Summary:
The assertion that both functions were not missing was incorrect and would
fail when one of the functions was missing. Fixed it and moved the
assertion earlier to check the input parameters to better capture
first-failure.  Added lit test.

Author: Jamie Schmeiser <schmeise@ca.ibm.com>
Reviewed By: aeubanks (Arthur Eubanks)
Differential Revision: https://reviews.llvm.org/D107989

llvm/lib/Passes/StandardInstrumentations.cpp

@@ -647,13 +647,12 @@ void ChangedIRComparer::compare(Any IR, StringRef Prefix, StringRef PassID,
 
   ChangedIRData::report(
       Before, After, [&](const ChangedFuncData *B, const ChangedFuncData *A) {
+        assert((B || A) && "Both functions cannot be missing.");
         ChangedFuncData Missing;
         if (!B)
           B = &Missing;
         else if (!A)
           A = &Missing;
-        assert(B != &Missing && A != &Missing &&
-               "Both functions cannot be missing.");
         handleFunctionCompare(Name, Prefix, PassID, true, *B, *A);
       });
 }

llvm/test/Other/ChangePrinters/print-changed-D107989.ll

@@ -0,0 +1,13 @@
+; D107989 This triggered an assert
+
+; RUN: opt  -passes=globalopt < %s -disable-output -print-changed=diff-quiet
+
+define signext i32 @main()  {
+entry:
+  ret i32 0
+}
+
+define internal void @f() {
+entry:
+  ret void
+}