[llvm] [IR] Add support for ignoring poisons in `getSplatValue` (PR #89155)

[via llvm-commits]

goldsteinn wrote:

> I don't think this is the change we want to do. I've posted an alternative at #89159.
> 
> Basically:
> 
> * In the spirit of [[PatternMatch] Do not accept undef elements in m_AllOnes() and friends #88217](https://github.com/llvm/llvm-project/pull/88217), we should remove support for undef splats to the degree possible, not support both.
> * Allowing poison splats by default in m_APInt/m_APFloat is a separate change, and requires at least some due diligence (at least a cursory review of select combines, where this is most likely to cause issues.)

Yeah I'm happy with that. The goal was poison support in getsplat, don't have any particular attachment/reason for keeping the undef stuff.

https://github.com/llvm/llvm-project/pull/89155