[llvm] [x86] Add lowering for `@llvm.experimental.vector.compress` (PR #104904)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

You can merge the two files with something ala:
```
# RUN: llc -o - -mtriple=aarch64-unknown-unknown -run-pass=aarch64-prelegalizer-combiner -verify-machineinstrs  %s | FileCheck %s --check-prefixes=CHECK,CHECK-AVX2
# RUN: llc -o - -mtriple=aarch64-unknown-unknown -run-pass=aarch64-postlegalizer-combiner -verify-machineinstrs  %s | FileCheck %s --check-prefixes=CHECK,CHECK-AVX512
```


https://github.com/llvm/llvm-project/pull/104904