[llvm] [VPlan] Introduce explicit broadcasts for live-ins. (PR #124644)

[Mel Chen via llvm-commits]

https://github.com/Mel-Chen approved this pull request.

Thank you! This is a great first step for explicit splat.
LGTM if the comments from @david-arm is addressed.

https://github.com/llvm/llvm-project/pull/124644