[RISCV] Return false from isShuffleMaskLegal except for splats.

We don't support any other shuffles currently.

This changes the bswap/bitreverse tests that check for this in
their expansion code. Previously we expanded a byte swapping
shuffle through memory. Now we're scalarizing and doing bit
operations on scalars to swap bytes.

In the future we can probably use vrgather.vx to do a byte swap
shuffle.

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -1068,6 +1068,14 @@ bool RISCVTargetLowering::shouldExpandBuildVectorWithShuffles(
   return false;
 }
 
+bool RISCVTargetLowering::isShuffleMaskLegal(ArrayRef<int> M, EVT VT) const {
+  // Only splats are currently supported.
+  if (ShuffleVectorSDNode::isSplatMask(M.data(), VT))
+    return true;
+
+  return false;
+}
+
 static SDValue lowerBUILD_VECTOR(SDValue Op, SelectionDAG &DAG,
                                  const RISCVSubtarget &Subtarget) {
   MVT VT = Op.getSimpleValueType();

llvm/lib/Target/RISCV/RISCVISelLowering.h

@@ -250,6 +250,10 @@ public:
   bool isFPImmLegal(const APFloat &Imm, EVT VT,
                     bool ForCodeSize) const override;
 
+  /// Return true if the given shuffle mask can be codegen'd directly, or if it
+  /// should be stack expanded.
+  bool isShuffleMaskLegal(ArrayRef<int> M, EVT VT) const override;
+
   bool hasBitPreservingFPLogic(EVT VT) const override;
   bool
   shouldExpandBuildVectorWithShuffles(EVT VT,