[ISEL] Move trivial step_vector folds to FoldConstantArithmetic.

[sdesmalen]

Summary

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.

[david-arm]

LGTM!

[sdesmalen]

Readded code for the original combines, because in a different patch
I realised that if a node's operands get updated using ReplaceAllUses,
the updated node may again become a candidate for a fold, even though
it doesn't reach the code in getNode() (i.e. no new node is created).

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

if (BV1 && BV2 && BVVT.isInteger() && BVVT == N2.getValueType()) {
             Ops[I] = getConstant(DstBits[I].sextOrSelf(BVEltBits), DL, BVEltVT);
           }
           return getBitcast(VT, getBuildVector(BVVT, DL, Ops));
         }
       }
     }
   }
 
+  // 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;
   };
 
   auto IsBuildVectorSplatVectorOrUndef = [](const SDValue &Op) {
     return Op.isUndef() || Op.getOpcode() == ISD::CONDCODE ||
            Op.getOpcode() == ISD::BUILD_VECTOR ||