[llvm] [VPlan] Introduce VPConstant VPIRValue (NFC) (PR #207387)

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

LGTM

> The motivation of introducing this new class is to refine optimizations to account for the fact that live-ins need broadcast.

I think this should specifically say "non-constant live-ins need broadcast"?

https://github.com/llvm/llvm-project/pull/207387