[LV] Extend dead instruction detection to multiple exiting blocks

Given we haven't yet enabled multiple exiting blocks, this is currently non functional, but it's an obvious extension which cleans up a later patch.

I don't think this is worth review (as it's pretty obvious), if anyone disagrees, feel feel to revert or comment and I will.

Author: preames

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7472,16 +7472,23 @@ void LoopVectorizationPlanner::executePlan(InnerLoopVectorizer &ILV,
 
 void LoopVectorizationPlanner::collectTriviallyDeadInstructions(
     SmallPtrSetImpl<Instruction *> &DeadInstructions) {
-  BasicBlock *Latch = OrigLoop->getLoopLatch();
 
-  // We create new control-flow for the vectorized loop, so the original
-  // condition will be dead after vectorization if it's only used by the
-  // branch.
-  auto *Cmp = dyn_cast<Instruction>(Latch->getTerminator()->getOperand(0));
-  if (Cmp && Cmp->hasOneUse()) {
-    DeadInstructions.insert(Cmp);
+  // We create new control-flow for the vectorized loop, so the original exit
+  // conditions will be dead after vectorization if it's only used by the
+  // terminator
+  SmallVector<BasicBlock*> ExitingBlocks;
+  OrigLoop->getExitingBlocks(ExitingBlocks);
+  for (auto *BB : ExitingBlocks) {
+    auto *Cmp = dyn_cast<Instruction>(BB->getTerminator()->getOperand(0));
+    if (!Cmp || !Cmp->hasOneUse())
+      continue;
+
+    // TODO: we should introduce a getUniqueExitingBlocks on Loop
+    if (!DeadInstructions.insert(Cmp).second)
+      continue;
 
     // The operands of the icmp is often a dead trunc, used by IndUpdate.
+    // TODO: can recurse through operands in general
     for (Value *Op : Cmp->operands()) {
       if (isa<TruncInst>(Op) && Op->hasOneUse())
           DeadInstructions.insert(cast<Instruction>(Op));
@@ -7491,6 +7498,7 @@ void LoopVectorizationPlanner::collectTriviallyDeadInstructions(
   // We create new "steps" for induction variable updates to which the original
   // induction variables map. An original update instruction will be dead if
   // all its users except the induction variable are dead.
+  auto *Latch = OrigLoop->getLoopLatch();
   for (auto &Induction : Legal->getInductionVars()) {
     PHINode *Ind = Induction.first;
     auto *IndUpdate = cast<Instruction>(Ind->getIncomingValueForBlock(Latch));