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

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cmtice wrote:

This PR deleted the only use of the variable "Plan" in VPlanVerifier.cpp, defined on line 175. It was used in the call to vputils::isHeaderMask on line 194, but this PR eliminated that call. Please also remove the definition, as it is causing repeated failures of this buildbot: https://lab.llvm.org/buildbot/#/builders/228/builds/4243

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