[llvm] [RISCV] Decompose single source shuffles (without exact VLEN) (PR #126108)

[Alex Bradbury via llvm-commits]

asb wrote:

Bug posted https://github.com/llvm/llvm-project/issues/126943 - I have to step away from the computer right now so can't do the obvious steps of reducing further. Thanks for the help with the quick revert!

https://github.com/llvm/llvm-project/pull/126108