[llvm] [PowerPC] Use rldimi/rlwimi to optimize build_vector (PR #67640)

[Nemanja Ivanovic via llvm-commits]

nemanjai wrote:

> Is this still relevant?

It seems like it would be useful, but neither myself nor the author (I believe) work on PPC any longer. Perhaps this is of interest to @lei137 or @RolandF77?

https://github.com/llvm/llvm-project/pull/67640