[VPlan] Remove VPIRPhis in exit blocks when deleting scalar loop BBs.

DeleteDeadBlocks will remove single-entry phis. Remove them from the exit
VPIRBBs in VPlan as well, otherwise we would retain references to deleted
IR instructions.

Fixes MSan failures after 8907b6d
https://lab.llvm.org/buildbot/#/builders/164/builds/14013

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -978,6 +978,16 @@ void VPlan::execute(VPTransformState *State) {
 
   // If the original loop is unreachable, delete it and all its blocks.
   if (!ScalarPhVPBB->hasPredecessors()) {
+    // DeleteDeadBlocks will remove single-entry phis. Remove them from the exit
+    // VPIRBBs in VPlan as well, otherwise we would retain references to deleted
+    // IR instructions.
+    for (VPIRBasicBlock *EB : getExitBlocks()) {
+      for (VPRecipeBase &R : make_early_inc_range(EB->phis())) {
+        if (R.getNumOperands() == 1)
+          R.eraseFromParent();
+      }
+    }
+
     Loop *OrigLoop =
         State->LI->getLoopFor(getScalarHeader()->getIRBasicBlock());
     auto Blocks = OrigLoop->getBlocksVector();