[llvm] [SelectionDAG] Use ZERO_EXTEND_VECTOR_INREG for vectors (PR #92048)

[Craig Topper via llvm-commits]

topperc wrote:

> I don't think this code path is exercised. I put an assert(0) in it and nothing failed.

New test coverage [e417e61532ac373e7b0708262dedefcdaf6ced9c](https://github.com/llvm/llvm-project/commit/e417e61532ac373e7b0708262dedefcdaf6ced9c)

https://github.com/llvm/llvm-project/pull/92048