[llvm] [DAG] isKnownNeverZero - add DemandedElts argument (PR #182679)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Closes #181656 .
> 
> Following changes were made for isKnownNeverZero :
> 
> * Added BUILDVECTOR and SPLATVECTOR cases.
> * Added support for DemandedElts arguments for SELECT/VSELECT cases.
> * Added tests for constants and SELECT/VSELECT.

This looks like what should be 2 different changes. Also lit tests are greatly preferably to unit tests 


https://github.com/llvm/llvm-project/pull/182679