[llvm] [VPlan] Fix crash when unrolling in-loop reduction chains (PR #129840)

[Florian Hahn via llvm-commits]

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@@ -66,6 +66,88 @@ entry:
   ret i32 %sum.0.lcssa
 }
 
+define i64 @reduction_sum_double(ptr noalias %p, ptr noalias %q) {
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fhahn wrote:

Could you add a brief comment here to say that this checks 2 reductions chained together or update the name? Not sure if `_double` makes this entirely clear

https://github.com/llvm/llvm-project/pull/129840