[RISCV] Simplify scalable vector case in lowerVectorMaskExt.

[craig.topper]

Summary

Since we have SPLAT_VECTOR_PARTS these days, I don't think we need
to go through extra lengths to avoid introducing an illegal scalar type.
We can just call getConstant using the scalable vector type and let
it create either a SPLAT_VECTOR or a SPLAT_VECTOR_PARTS.

[rogfer01]

LGTM. Thanks for the cleanup @craig.topper

[frasercrmck]

Ach I hadn't pressed submit on my acceptance, sorry. LGTM too

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

 //===-- RISCVISelLowering.cpp - RISCV DAG Lowering Implementation  --------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
SDValue RISCVTargetLowering::lowerVectorMaskExt(SDValue Op, SelectionDAG &DAG,
                                                 int64_t ExtTrueVal) const {
   SDLoc DL(Op);
   MVT VecVT = Op.getSimpleValueType();
   SDValue Src = Op.getOperand(0);
   // Only custom-lower extensions from mask types
   assert(Src.getValueType().isVector() &&
          Src.getValueType().getVectorElementType() == MVT::i1);
 
-  MVT XLenVT = Subtarget.getXLenVT();
-  SDValue SplatZero = DAG.getConstant(0, DL, XLenVT);
-  SDValue SplatTrueVal = DAG.getConstant(ExtTrueVal, DL, XLenVT);
-
   if (VecVT.isScalableVector()) {
-    // Be careful not to introduce illegal scalar types at this stage, and be
-    // careful also about splatting constants as on RV32, vXi64 SPLAT_VECTOR is
-    // illegal and must be expanded. Since we know that the constants are
-    // sign-extended 32-bit values, we use VMV_V_X_VL directly.
-    bool IsRV32E64 =
-        !Subtarget.is64Bit() && VecVT.getVectorElementType() == MVT::i64;
-
-    if (!IsRV32E64) {
-      SplatZero = DAG.getSplatVector(VecVT, DL, SplatZero);
-      SplatTrueVal = DAG.getSplatVector(VecVT, DL, SplatTrueVal);
-    } else {
-      SplatZero =
-          DAG.getNode(RISCVISD::VMV_V_X_VL, DL, VecVT, DAG.getUNDEF(VecVT),
-                      SplatZero, DAG.getRegister(RISCV::X0, XLenVT));
-      SplatTrueVal =
-          DAG.getNode(RISCVISD::VMV_V_X_VL, DL, VecVT, DAG.getUNDEF(VecVT),
-                      SplatTrueVal, DAG.getRegister(RISCV::X0, XLenVT));
-    }
-
+    SDValue SplatZero = DAG.getConstant(0, DL, VecVT);
+    SDValue SplatTrueVal = DAG.getConstant(ExtTrueVal, DL, VecVT);
     return DAG.getNode(ISD::VSELECT, DL, VecVT, Src, SplatTrueVal, SplatZero);
   }
 
   MVT ContainerVT = getContainerForFixedLengthVector(VecVT);
   MVT I1ContainerVT =
       MVT::getVectorVT(MVT::i1, ContainerVT.getVectorElementCount());
 
   SDValue CC = convertToScalableVector(I1ContainerVT, Src, DAG, Subtarget);
 
   SDValue Mask, VL;
   std::tie(Mask, VL) = getDefaultVLOps(VecVT, ContainerVT, DL, DAG, Subtarget);
 
+  MVT XLenVT = Subtarget.getXLenVT();
+  SDValue SplatZero = DAG.getConstant(0, DL, XLenVT);
+  SDValue SplatTrueVal = DAG.getConstant(ExtTrueVal, DL, XLenVT);
+
   SplatZero = DAG.getNode(RISCVISD::VMV_V_X_VL, DL, ContainerVT,
                           DAG.getUNDEF(ContainerVT), SplatZero, VL);
   SplatTrueVal = DAG.getNode(RISCVISD::VMV_V_X_VL, DL, ContainerVT,
                              DAG.getUNDEF(ContainerVT), SplatTrueVal, VL);
   SDValue Select = DAG.getNode(RISCVISD::VSELECT_VL, DL, ContainerVT, CC,
                                SplatTrueVal, SplatZero, VL);
 
   return convertFromScalableVector(VecVT, Select, DAG, Subtarget);