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

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

I've added some X86 and RISC-V tests now. For RISC-V it looks like compresstore/expandload is only legal for fixed VFs, so we must add `-force-vector-width=4` to the RUN line. 

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