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

[via llvm-commits]

github-actions[bot] wrote:

test-suite diff from 34152bb7d2f37e4bedd6eb9b06a21ae54211aedc...6bc433d87bbf17a7689b63817c6afdc7b0a85e95: https://github.com/llvm/llvm-project/actions/runs/35856323246/artifacts/10753101943

Statistics: https://github.com/llvm/llvm-project/actions/runs/35856323246/artifacts/10752997163

## riscv64-linux-gnu `-march=rva23u64 -O3`
| ('Program', '')    |   ('size..text', 'base') |   ('size..text', 'head') |   ('size..text', 'diff') |
|:-------------------|-------------------------:|-------------------------:|-------------------------:|
| Geomean difference |                      nan |                      nan |                        0 |
## aarch64-linux-gnu `-march=armv9-a -O3`
| ('Program', '')                             |   ('size..text', 'base') |   ('size..text', 'head') |   ('size..text', 'diff') |
|:--------------------------------------------|-------------------------:|-------------------------:|-------------------------:|
| SingleSour...nchmarks/CoyoteBench/huffbench |                     3000 |                     3024 |              0.008       |
| MultiSourc...ch/consumer-jpeg/consumer-jpeg |                   151068 |                   151200 |              0.000873779 |
| MultiSourc.../mediabench/jpeg/jpeg-6a/cjpeg |                   155048 |                   155180 |              0.000851349 |
| MultiSourc.../MallocBench/espresso/espresso |                   151580 |                   151672 |              0.00060694  |
| Geomean difference                          |                      nan |                      nan |              3.19859e-05 |
## x86_64-linux-gnu `-march=x86-64-v3 -O3`
| ('Program', '')    |   ('size..text', 'base') |   ('size..text', 'head') |   ('size..text', 'diff') |
|:-------------------|-------------------------:|-------------------------:|-------------------------:|
| Geomean difference |                      nan |                      nan |                        0 |


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