[llvm] [RISCV] Add lowerVECTOR_SHUFFLEAsCONCAT_VECTORS. (PR #109948)

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

The PR could use a small description of the transform but otherwise LGTM

https://github.com/llvm/llvm-project/pull/109948