[llvm] [PatternMatch] Do not accept undef elements in m_AllOnes() and friends (PR #88217)

[Nikita Popov via llvm-commits]

nikic wrote:

Okay, this should be ready for another round of review now. I've updated all the affected tests now (mostly by replacing undef -> poison, in some cases by duplicating tests where I thought it had value).

I've also made some minor code cleanup while doing the test updates. The main one is that I dropped the m_NotForbidUndef matcher, because all uses of it are fine with poison elements.

https://github.com/llvm/llvm-project/pull/88217