[llvm] [SelectionDAG] Add computeKnownBits support for ISD::STEP_VECTOR (PR #80452)

[Philip Reames via llvm-commits]

================
@@ -3110,6 +3110,33 @@ KnownBits SelectionDAG::computeKnownBits(SDValue Op, const APInt &DemandedElts,
     }
     break;
   }
+  case ISD::STEP_VECTOR: {
+    const APInt &Step = Op.getConstantOperandAPInt(0);
+
+    if (Step.isPowerOf2())
+      Known.Zero.setLowBits(Step.logBase2());
+
+    const Function &F = getMachineFunction().getFunction();
+
+    if (!isUIntN(BitWidth, Op.getValueType().getVectorMinNumElements()))
+      break;
+    const APInt MinNumElts =
+        APInt(BitWidth, Op.getValueType().getVectorMinNumElements());
+
+    bool Overflow;
+    const APInt MaxNumElts = getVScaleRange(&F, BitWidth)
----------------
preames wrote:

Never mind, we have the assert in SelectionDAG::getNode
```
   assert(VT.isScalableVector() &&
           "STEP_VECTOR can only be used with scalable types");
    assert(OpOpcode == ISD::TargetConstant &&
           VT.getVectorElementType() == N1.getValueType() &&
           "Unexpected step operand");
```

https://github.com/llvm/llvm-project/pull/80452