[llvm] [LoopVectorize] Support vectorization of compressing patterns (PR #214491)

[Luke Lau via llvm-commits]

lukel97 wrote:

> > Can you add a RISC-V test too? It can lower the compressed store/load intrinsics, would be good to check that the TTI is plumbed up correctly for it as well
> 
> Do you have a suggested triple / target-features to use?

I think `-mtriple riscv64 -mattr=+v` should be enough for RISC-V

https://github.com/llvm/llvm-project/pull/214491