[llvm] [LV] Fix unsafe canVectorizeWithIfConvert() checks (PR #203273)

[Gaëtan Bossu via llvm-commits]

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@@ -2962,7 +2962,10 @@ bool LoopAccessInfo::blockNeedsPredication(const BasicBlock *BB,
   assert(TheLoop->contains(BB) && "Unknown block used");
 
   // Blocks that do not dominate the latch need predication.
+  // Treat blocks in loops with no unique latch as always needing predication.
   const BasicBlock *Latch = TheLoop->getLoopLatch();
+  if (!Latch)
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gbossu wrote:

I chose `return true` as the conservative value given how `blockNeedsPredication` is used in LV.

If you prefer, I could maybe fix the callsites instead and make `TheLoop->getLoopLatch() != nullptr` a precondition for `blockNeedsPredication`.

https://github.com/llvm/llvm-project/pull/203273