[RISCV] Use Align(8) for the stack temporary created for SPLAT_VECTOR_SPLIT_I64_VL.

The value needs to be read as an 8 byte vector element which requires
the pointer to be 8 byte aligned according to the vector spec.

Fixes #71787

Author: topperc

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

@@ -92,7 +92,7 @@ void RISCVDAGToDAGISel::PreprocessISelDAG() {
 
       // Create temporary stack for each expanding node.
       SDValue StackSlot =
-          CurDAG->CreateStackTemporary(TypeSize::Fixed(8), Align(4));
+          CurDAG->CreateStackTemporary(TypeSize::Fixed(8), Align(8));
       int FI = cast<FrameIndexSDNode>(StackSlot.getNode())->getIndex();
       MachinePointerInfo MPI = MachinePointerInfo::getFixedStack(MF, FI);
 

llvm/test/CodeGen/RISCV/intrinsic-cttz-elts-vscale.ll

@@ -75,9 +75,9 @@ define i64 @ctz_nxv8i1_no_range(<vscale x 8 x i16> %a) {
 ; RV32-NEXT:    li a1, 0
 ; RV32-NEXT:    li a3, 0
 ; RV32-NEXT:    call __muldi3@plt
-; RV32-NEXT:    sw a1, 24(sp)
-; RV32-NEXT:    sw a0, 20(sp)
-; RV32-NEXT:    addi a2, sp, 20
+; RV32-NEXT:    sw a1, 20(sp)
+; RV32-NEXT:    sw a0, 16(sp)
+; RV32-NEXT:    addi a2, sp, 16
 ; RV32-NEXT:    vsetvli a3, zero, e64, m8, ta, ma
 ; RV32-NEXT:    vlse64.v v16, (a2), zero
 ; RV32-NEXT:    vid.v v8