[llvm] [LV] Don't predicate divs with invariant divisor when folding tail (PR #98904)

[via llvm-commits]

================
@@ -3348,40 +3348,48 @@ bool LoopVectorizationCostModel::isPredicatedInst(Instruction *I) const {
     return false;
 
   // Can we prove this instruction is safe to unconditionally execute?
-  // If not, we must use some form of predication.
+  if (I->getOpcode() == Instruction::Call)
+    return Legal->isMaskRequired(I);
+
+  if (isa<LoadInst, StoreInst>(I) && !Legal->isMaskRequired(I))
+    return false;
+
+  // TODO: We can use the loop-preheader as context point here and get
+  // context sensitive reasoning
+  if (isa<BranchInst, PHINode>(I) || isSafeToSpeculativelyExecute(I))
+    return false;
+
+  // If the instruction was executed conditionally in the original scalar loop,
+  // predication is needed.
+  if (Legal->blockNeedsPredication(I->getParent()))
+    return true;
+
+  // Tail folding may introduce additional predication, but we're guaranteed to
+  // always have at least one active lane. If the instruction in the original
+  // scalar loop was executed unconditionally, it may not need predication,
+  // depending on its operands.
   switch(I->getOpcode()) {
   default:
-    return false;
+    llvm_unreachable(
+        "instruction should have been considered to not require predication "
+        "by earlier checks");
----------------
ayalz wrote:

```suggestion
        "instruction should have been considered by earlier checks");
```
to either require predication or not.

https://github.com/llvm/llvm-project/pull/98904