[llvm] [CodeGenPrepare] don't promote sdiv/srem by -1 (PR #218737)

[via llvm-commits]

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@@ -8367,16 +8367,13 @@ class VectorPromoteHelper {
   /// Check if it is profitable to promote \p ToBePromoted
   /// by moving downward the transition through.
   bool shouldPromote(const Instruction *ToBePromoted) const {
+    if (!isSafeToSpeculativelyExecuteWithVariableReplaced(ToBePromoted))
+      return false;
     // Promote only if all the operands can be statically expanded.
     // Indeed, we do not want to introduce any new kind of transitions.
     for (const Use &U : ToBePromoted->operands()) {
       const Value *Val = U.get();
       if (Val == getEndOfTransition()) {
-        // If the use is a division and the transition is on the rhs,
-        // we cannot promote the operation, otherwise we may create a
-        // division by zero.
-        if (canCauseUndefinedBehavior(ToBePromoted, U.getOperandNo()))
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im-lunex wrote:

https://github.com/llvm/llvm-project/blob/58e77af2bbdd95632a03787737627dda52e9cf07/llvm/lib/CodeGen/CodeGenPrepare.cpp#L8507-L8510

Here I tried using `isSafeToSpeculativelyExecuteWithVariableReplaced()` but since it only checks if the divisor is a 0 and has no concept of vector `poison`, it caused issues in this specific case. Instead, using `(U.getOperandNo() == 1 && ToBePromoted->isIntDivRem())` just prevents crash where it is needed .. this might change the behavior a bit .. i think this change should be okay.

https://github.com/llvm/llvm-project/pull/218737