[MBP] Prevent rotating a chain contains entry block

The entry block should always be the first BB in a function.
So we should not rotate a chain contains the entry block.

Differential Revision: https://reviews.llvm.org/D92882

llvm/lib/CodeGen/MachineBlockPlacement.cpp

@@ -2306,6 +2306,10 @@ void MachineBlockPlacement::rotateLoop(BlockChain &LoopChain,
   if (Bottom == ExitingBB)
     return;
 
+  // The entry block should always be the first BB in a function.
+  if (Top->isEntryBlock())
+    return;
+
   bool ViableTopFallthrough = hasViableTopFallthrough(Top, LoopBlockSet);
 
   // If the header has viable fallthrough, check whether the current loop
@@ -2380,6 +2384,11 @@ void MachineBlockPlacement::rotateLoopWithProfile(
     BlockChain &LoopChain, const MachineLoop &L,
     const BlockFilterSet &LoopBlockSet) {
   auto RotationPos = LoopChain.end();
+  MachineBasicBlock *ChainHeaderBB = *LoopChain.begin();
+
+  // The entry block should always be the first BB in a function.
+  if (ChainHeaderBB->isEntryBlock())
+    return;
 
   BlockFrequency SmallestRotationCost = BlockFrequency::getMaxFrequency();
 
@@ -2398,7 +2407,6 @@ void MachineBlockPlacement::rotateLoopWithProfile(
   // chain head is not the loop header. As we only consider natural loops with
   // single header, this computation can be done only once.
   BlockFrequency HeaderFallThroughCost(0);
-  MachineBasicBlock *ChainHeaderBB = *LoopChain.begin();
   for (auto *Pred : ChainHeaderBB->predecessors()) {
     BlockChain *PredChain = BlockToChain[Pred];
     if (!LoopBlockSet.count(Pred) &&