[llvm] [Vplan] Tighten m_CanonicalIV() (PR #194312)

[Ramkumar Ramachandra via llvm-commits]

================

----------------
artagnon wrote:

Yes, that would be a positive change, and would make the current change testable -- perhaps put up the header mask change, remove the non-corresponding test in the current PR, and add the test to the header mask change?

https://github.com/llvm/llvm-project/pull/194312