[llvm] [SelectionDAG] Add expansion for llvm.convert.from.arbitrary (PR #179318)

[Dmitry Sidorov via llvm-commits]

MrSidims wrote:

@arsenm @RKSimon @kuhar @efriedma-quic @nikic friendly ping in case if you have missed it (as there was a github CI outage the day I pushed it). Also may be there are other people, who might be interested in - if so, please call them in the PR. 

https://github.com/llvm/llvm-project/pull/179318