[llvm] [SelectionDAG] Handle scalable INSERT_SUBVECTOR in ComputeNumSignBits (PR #221631)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> > Any chance of an IR test case instead of relying on unit tests?
> 
> And should I keep the existing unit tests as well?

I don't think so - you shouldn't need to if you have sufficient IR coverage

https://github.com/llvm/llvm-project/pull/221631