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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

x86 AVX512 commpresstore costs: #226240 (still waiting review) - annoyingly test-suite won't be much help for x86-64-v4 :( 

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