[SimpleLoopUnswitch] Preserve one PHI when removing a predecessor of a BB

Author: dianqk

llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp

@@ -1716,7 +1716,7 @@ static void deleteDeadBlocksFromLoop(Loop &L,
     auto *BB = DeathCandidates.pop_back_val();
     if (!DeadBlockSet.count(BB) && !DT.isReachableFromEntry(BB)) {
       for (BasicBlock *SuccBB : successors(BB)) {
-        SuccBB->removePredecessor(BB);
+        SuccBB->removePredecessor(BB, /*KeepOneInputPHIs*/ true);
         DeathCandidates.push_back(SuccBB);
       }
       DeadBlockSet.insert(BB);

llvm/test/Transforms/SimpleLoopUnswitch/nontrivial-unswitch-pred-removed.ll

@@ -1,8 +1,12 @@
 ; RUN: opt -passes='loop-mssa(simple-loop-unswitch<nontrivial>),print<memoryssa>' -verify-memoryssa -disable-output -S < %s 2>&1 | FileCheck %s
+; RUN: opt -passes='loop-mssa(simple-loop-unswitch<nontrivial>,licm)' -verify-memoryssa -disable-output -S 
 
-; CHECK: preds = %bb2{{$}}
+; Check that SimpleLoopUnswitch preserves the MemoryDef of `call i32 @bar()` by preserving the PHI node.
+; Also, check that executing LICM after SimpleLoopUnswitch does not result in a crash.
+
+; CHECK: %unswitched.select = phi ptr [ @bar, %bb2 ]
 ; CHECK-NEXT: MemoryDef
-; CHECK-NEXT: call i32 @bar()
+; CHECK-NEXT: call i32 %unswitched.select()
 
 define i32 @foo(i1 %arg, ptr %arg1) {
 bb: