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

[Simon Pilgrim via llvm-commits]

================
@@ -2321,6 +2321,29 @@ X86TargetLowering::X86TargetLowering(const X86TargetMachine &TM,
     }
   }
 
+  // vpcompress depends on various AVX512 extensions.
+  if (Subtarget.hasAVX512()) {
----------------
RKSimon wrote:

Move these into the `if (!Subtarget.useSoftFloat() && Subtarget.hasAVX512())` block earlier

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