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

[Luke Lau via llvm-commits]

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

LGTM. Would be good to document somewhere the definition of the header mask as specifically (`canIV < TC`), since previous `findHeaderMask` also used to include the alias mask too

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