[llvm] [VPlan] Rename onlyFirst(Lane|Part)Used (NFC) (PR #166562)

[Ramkumar Ramachandra via llvm-commits]

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@@ -18,7 +18,7 @@ using namespace llvm::VPlanPatternMatch;
 
 bool vputils::onlyFirstLaneUsed(const VPValue *Def) {
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artagnon wrote:

One of the motivations for the rename is to differentiate it from this function: onlyFirstLaneUsed says "only first lane is used by users", while usesFirstLaneOnly says "this only uses the first lane of \p Op".

https://github.com/llvm/llvm-project/pull/166562