[llvm] [VPlan] Mark histogram increment as only having its first lane used. (PR #226138)

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@llvm/pr-subscribers-llvm-transforms

Author: Florian Hahn (fhahn)

<details>
<summary>Changes</summary>

The second operand (increment ) of llvm.experimental.vector.histogram.* is a scalar. Handle it in usesFirstLaneOnly.

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Full diff: https://github.com/llvm/llvm-project/pull/226138.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/Vectorize/VPlan.h (+7) 
- (modified) llvm/test/Transforms/LoopVectorize/AArch64/sve2-histcnt.ll (+1-4) 


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https://github.com/llvm/llvm-project/pull/226138