[llvm] [AArch64][CostModel] Lower cost of dupq (SVE2.1) (PR #144918)

[Sander de Smalen via llvm-commits]

================
@@ -29981,7 +29981,8 @@ SDValue AArch64TargetLowering::LowerFixedLengthVECTOR_SHUFFLEToSVE(
           DAG, VT, DAG.getNode(Opc, DL, ContainerVT, Op1, Op1));
     }
 
-    if (Subtarget->hasSVE2p1() || Subtarget->hasSME2p1()) {
+    if (Subtarget->hasSVE2p1() ||
+        (Subtarget->hasSME2p1() && Subtarget->isSVEorStreamingSVEAvailable())) {
----------------
sdesmalen-arm wrote:

After having revisited this once more, the correct interpretation of:
```
if !Feature_SVE2p1 && !Feature_SME2p1 then Instruction = UNDEFINED

CheckSVEEnabled()
  // can be in streaming mode
  // can be in non-streaming mode
```

Is that:
* `+sve,+sme2p1` enables DUPQ in both streaming and non-streaming mode
* `+sve2p1,+sme` enables DUPQ in both streaming and non-streaming mode

Meaning that for DUPQ this expression should be:
```
if (Subtarget->hasSVE2p1() || Subtarget->hasSME2p1()) &&
    Subtarget->isSVEorStreamingSVEAvailable())
```

We previously worked under the assumption that +sme* features would add features in streaming mode, and +sve* features would add features in non-streaming mode. But this is not (or no longer) the case. There may be other places where this needs fixing as well.

https://github.com/llvm/llvm-project/pull/144918