[ISEL] Move trivial step_vector folds to FoldConstantArithmetic.

Given that step_vector is practically a constant, doing this early
helps with DAGCombine folds that happen before type legalization.

There is currently no way to test this happens earlier, although existing
tests for step_vector folds continue protect the folds happening at all.

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D117863

Author: sdesmalen-arm

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -5418,6 +5418,19 @@ SDValue SelectionDAG::FoldConstantArithmetic(unsigned Opcode, const SDLoc &DL,
     }
   }
 
+  // Fold (mul step_vector(C0), C1) to (step_vector(C0 * C1)).
+  //      (shl step_vector(C0), C1) -> (step_vector(C0 << C1))
+  if ((Opcode == ISD::MUL || Opcode == ISD::SHL) &&
+      Ops[0].getOpcode() == ISD::STEP_VECTOR) {
+    APInt RHSVal;
+    if (ISD::isConstantSplatVector(Ops[1].getNode(), RHSVal)) {
+      APInt NewStep = Opcode == ISD::MUL
+                          ? Ops[0].getConstantOperandAPInt(0) * RHSVal
+                          : Ops[0].getConstantOperandAPInt(0) << RHSVal;
+      return getStepVector(DL, VT, NewStep);
+    }
+  }
+
   auto IsScalarOrSameVectorSize = [NumElts](const SDValue &Op) {
     return !Op.getValueType().isVector() ||
            Op.getValueType().getVectorElementCount() == NumElts;