[llvm] [AArch64] Combine PTEST_FIRST(PTRUE, CONCAT(A, B)) -> PTEST_FIRST(PTRUE, A) (PR #161384)

[Paul Walker via llvm-commits]

================
@@ -27519,6 +27522,37 @@ static SDValue performMULLCombine(SDNode *N,
   return SDValue();
 }
 
+static SDValue performPTestFirstCombine(SDNode *N,
+                                        TargetLowering::DAGCombinerInfo &DCI,
+                                        SelectionDAG &DAG) {
+  if (DCI.isBeforeLegalize())
+    return SDValue();
+
+  SDLoc DL(N);
+  auto Mask = N->getOperand(0);
+  auto Pred = N->getOperand(1);
+
+  if (Mask->getOpcode() == AArch64ISD::REINTERPRET_CAST)
+    Mask = Mask->getOperand(0);
+
+  if (Pred->getOpcode() == AArch64ISD::REINTERPRET_CAST)
+    Pred = Pred->getOperand(0);
+
+  if (Pred->getValueType(0).getVectorElementType() != MVT::i1 ||
+      !isAllActivePredicate(DAG, Mask))
+    return SDValue();
+
+  if (Pred->getOpcode() == ISD::CONCAT_VECTORS) {
+    Pred = Pred->getOperand(0);
+    SDValue Mask = DAG.getSplatVector(Pred->getValueType(0), DL,
+                                      DAG.getAllOnesConstant(DL, MVT::i64));
----------------
paulwalker-arm wrote:

You've already proven the existing `Mask` does what you need so should be reusable.  Perhaps it's worth adding an `isLanes1toNKnownOne` helper function, then you don't need to strip reinterpret from `Mask` and can then reuse it directly.

https://github.com/llvm/llvm-project/pull/161384