[llvm] [VPlan] Consistently use (Part, 0) for first lane scalar values (PR #80271)

[Florian Hahn via llvm-commits]

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@@ -526,12 +542,13 @@ bool VPInstruction::onlyFirstLaneUsed(const VPValue *Op) const {
   case Instruction::ICmp:
     // TODO: Cover additional opcodes.
     return vputils::onlyFirstLaneUsed(this);
+  case VPInstruction::ComputeReductionResult:
+    return true;
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fhahn wrote:

Removed ComputeReductionResult change here as it is no needed. Can split off if preferred, the code just won't be exercised by any tests until this patch.

https://github.com/llvm/llvm-project/pull/80271