[SelectionDAG][RISCV] Teach getConstant to use SPLAT_VECTOR_PARTS if vXi64 SPLAT_VECTOR is legal but i64 scalars are not.

That matches how such a SPLAT_VECTOR would have been type legalized
so assume it is ok to use for creating constants after type legalization.

Still need some improvements to SPLAT_VECTOR lowering.

This overlaps with some of what D158742 was trying to fix.

Reviewed By: luke

Differential Revision: https://reviews.llvm.org/D158870

Author: topperc

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -1631,7 +1631,8 @@ SDValue SelectionDAG::getConstant(const ConstantInt &Val, const SDLoc &DL,
     unsigned ViaEltSizeInBits = ViaEltVT.getSizeInBits();
 
     // For scalable vectors, try to use a SPLAT_VECTOR_PARTS node.
-    if (VT.isScalableVector()) {
+    if (VT.isScalableVector() ||
+        TLI->isOperationLegal(ISD::SPLAT_VECTOR, VT)) {
       assert(EltVT.getSizeInBits() % ViaEltSizeInBits == 0 &&
              "Can only handle an even split!");
       unsigned Parts = EltVT.getSizeInBits() / ViaEltSizeInBits;