Revert "[IR][LoopRotate] avoid leaving phi with no operands (PR48296)"

This reverts commit bfd2c21.
This appears to be causing stage2 msan failures on buildbots:
  FAIL: LLVM :: Transforms/SimplifyCFG/X86/bug-25299.ll (65872 of 71835)
  ******************** TEST 'LLVM :: Transforms/SimplifyCFG/X86/bug-25299.ll' FAILED ********************
  Script:
  --
  : 'RUN: at line 1';   /b/sanitizer-x86_64-linux-fast/build/llvm_build_msan/bin/opt < /b/sanitizer-x86_64-linux-fast/build/llvm-project/llvm/test/Transforms/SimplifyCFG/X86/bug-25299.ll -simplifycfg -S | /b/sanitizer-x86_64-linux-fast/build/llvm_build_msan/bin/FileCheck /b/sanitizer-x86_64-linux-fast/build/llvm-project/llvm/test/Transforms/SimplifyCFG/X86/bug-25299.ll
  --
  Exit Code: 2
  Command Output (stderr):
  --
  ==87374==WARNING: MemorySanitizer: use-of-uninitialized-value
      #0 0x9de47b6 in getBasicBlockIndex /b/sanitizer-x86_64-linux-fast/build/llvm-project/llvm/include/llvm/IR/Instructions.h:2749:5
      #1 0x9de47b6 in simplifyCommonResume /b/sanitizer-x86_64-linux-fast/build/llvm-project/llvm/lib/Transforms/Utils/SimplifyCFG.cpp:4112:23
      rust-lang#2 0x9de47b6 in simplifyResume /b/sanitizer-x86_64-linux-fast/build/llvm-project/llvm/lib/Transforms/Utils/SimplifyCFG.cpp:4039:12
      rust-lang#3 0x9de47b6 in (anonymous namespace)::SimplifyCFGOpt::simplifyOnce(llvm::BasicBlock*) /b/sanitizer-x86_64-linux-fast/build/llvm-project/llvm/lib/Transforms/Utils/SimplifyCFG.cpp:6330:16
      rust-lang#4 0x9dcca13 in run /b/sanitizer-x86_64-linux-fast/build/llvm-project/llvm/lib/Transforms/Utils/SimplifyCFG.cpp:6358:16
      rust-lang#5 0x9dcca13 in llvm::simplifyCFG(llvm::BasicBlock*, llvm::TargetTransformInfo const&, llvm::SimplifyCFGOptions const&, llvm::SmallPtrSetImpl<llvm::BasicBlock*>*) /b/sanitizer-x86_64-linux-fast/build/llvm-project/llvm/lib/Transforms/Utils/SimplifyCFG.cpp:6369:8
      rust-lang#6 0x974643d in iterativelySimplifyCFG(

Author: rotateright

llvm/include/llvm/IR/BasicBlock.h

@@ -387,9 +387,9 @@ class BasicBlock final : public Value, // Basic blocks are data objects also
   /// Update PHI nodes in this BasicBlock before removal of predecessor \p Pred.
   /// Note that this function does not actually remove the predecessor.
   ///
-  /// If \p KeepOneInputPHIs is true, then don't remove PHIs that are left with
-  /// one incoming value and don't simplify PHIs with all incoming values the
-  /// same.
+  /// If \p KeepOneInputPHIs is true then don't remove PHIs that are left with
+  /// zero or one incoming values, and don't simplify PHIs with all incoming
+  /// values the same.
   void removePredecessor(BasicBlock *Pred, bool KeepOneInputPHIs = false);
 
   bool canSplitPredecessors() const;

llvm/lib/IR/BasicBlock.cpp

@@ -330,7 +330,7 @@ void BasicBlock::removePredecessor(BasicBlock *Pred,
 
   unsigned NumPreds = cast<PHINode>(front()).getNumIncomingValues();
   for (PHINode &Phi : make_early_inc_range(phis())) {
-    Phi.removeIncomingValue(Pred);
+    Phi.removeIncomingValue(Pred, !KeepOneInputPHIs);
     if (KeepOneInputPHIs)
       continue;
     // If we have a single predecessor, removeIncomingValue erased the PHI