[PGO] Gracefully handle zero entry-count

With sampled instrumentation (#69535), profile counts can appear
corrupt.  In particular a function can have a 0 block count for its
entry, while later blocks are non zero.  This is only likely to happen
for colder functions, so it is reasonable to take any action that does
not crash.

llvm/lib/Transforms/Instrumentation/PGOInstrumentation.cpp

@@ -1970,11 +1970,9 @@ static void fixFuncEntryCount(PGOUseFunc &Func, LoopInfo &LI,
                               BranchProbabilityInfo &NBPI) {
   Function &F = Func.getFunc();
   BlockFrequencyInfo NBFI(F, NBPI, LI);
-#ifndef NDEBUG
   auto BFIEntryCount = F.getEntryCount();
-  assert(BFIEntryCount && (BFIEntryCount->getCount() > 0) &&
-         "Invalid BFI Entrycount");
-#endif
+  if (!BFIEntryCount || BFIEntryCount->getCount() == 0)
+    return;
   auto SumCount = APFloat::getZero(APFloat::IEEEdouble());
   auto SumBFICount = APFloat::getZero(APFloat::IEEEdouble());
   for (auto &BBI : F) {