[llvm] DAG: Fix vector_shuffle -> splat fold defining undef lanes (PR #123596)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

A few annoyances as it exposes more examples of known existing poor codegen on x86 - I'll see if there's anything we can do to avoid them

https://github.com/llvm/llvm-project/pull/123596