[llvm] [LangRef] Clarify semantics of undef in shufflevector masks (PR #171453)

[Nikita Popov via llvm-commits]

https://github.com/nikic approved this pull request.

LGTM, though we should probably actually make good on that "temporarily" at this point... We need to support this in bitcode, but we should probably start rejecting it in IR to avoid unnecessary confusion like this.

https://github.com/llvm/llvm-project/pull/171453