[llvm] [VPlan] Model initial header mask as region value. (PR #196199)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

> I think calculateRegisterUsageForPlan treats VPRegionValue as a scalar, that probably needs updated to check if it's the canonical IV specifically now

Currently we expand the header mask before using `calculateRegisterUsageForPlan`, but I updated it to directly check for the canonical IV, instead of checking VPRegionValue

https://github.com/llvm/llvm-project/pull/196199