[llvm] [LSR][term-fold] Ensure the simple recurrence is reachable from the current loop (PR #83085)

[Patrick O'Neill via llvm-commits]

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@@ -6808,6 +6808,12 @@ canFoldTermCondOfLoop(Loop *L, ScalarEvolution &SE, DominatorTree &DT,
   if (!matchSimpleRecurrence(LHS, ToFold, ToFoldStart, ToFoldStep))
     return std::nullopt;
 
+  // If ToFold does not have an incoming value from LoopLatch then the simple
+  // recurrence is from a prior loop unreachable from the loop we're currently
+  // considering.
+  if (ToFold->getBasicBlockIndex(LoopLatch) == -1)
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patrick-rivos wrote:

Updated.

Previously I had tried comparing with the LoopLatch which interfered with the SeveralLoopLatch testcase. I'm going to read up on how getHeader differs from LoopLatch. Thanks!

https://github.com/llvm/llvm-project/pull/83085