[llvm] [LLVM][LangRef] Remove "n > 0" restriction from get.active.lanes.mask. (PR #152140)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

See https://reviews.llvm.org/D86147 and https://reviews.llvm.org/D79175 for the original discussion of the restriction. CC @sjoerdmeijer @sparker-arm .

>From what I recall, there's some interaction with MVE tail predication that breaks with n=0.  But I haven't thought about this for a long time, so I'm not sure if that's still true.

https://github.com/llvm/llvm-project/pull/152140