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

[Simon Pilgrim via llvm-commits]

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+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc -mtriple=x86_64 -mattr=avx2 < %s | FileCheck %s
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RKSimon wrote:

Why do you have separate avx2 / avx512 test files? Just put both the RUN lines in a single file and use "--check-prefixes=AVX,AVX2" and "--check-prefixes=AVX,AVX512"

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