[llvm] [VN] be more consistent about forwarding null inputs and ignoring SVE outputs (PR #139574)

[Jameson Nash via llvm-commits]

vtjnash wrote:

> Does this change provide benefits for cases not involving aggregate types?

Probably rarely, this was mostly just for wanting to make the code more robust and consistent in handling them. Although the SROA pass does like making arrays of i8 as a means of creating an arbitrary slice of bytes and expects those to be just as well handled elsewhere as an integer or vector.

https://github.com/llvm/llvm-project/pull/139574