[RISCV] Treat __riscv_vsetvl_*(-1) as vlmax.

[craig.topper]

Summary

We already treat -1 passed to instruction intrinsics as vlmax, this
make vsetvli consistent.

[rogfer01]

LGTM. Thanks @craig.topper

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

RISCVII::VLMUL VLMul = static_cast<RISCVII::VLMUL>(
       Node->getConstantOperandVal(Offset + 1) & 0x7);
 
   unsigned VTypeI = RISCVVType::encodeVTYPE(VLMul, SEW, /*TailAgnostic*/ true,
                                             /*MaskAgnostic*/ true);
   SDValue VTypeIOp = CurDAG->getTargetConstant(VTypeI, DL, XLenVT);
 
   SDValue VLOperand;
   unsigned Opcode = RISCV::PseudoVSETVLI;
-  if (VLMax) {
+  if (VLMax || isAllOnesConstant(Node->getOperand(1))) {
     VLOperand = CurDAG->getRegister(RISCV::X0, XLenVT);
     Opcode = RISCV::PseudoVSETVLIX0;
   } else {
     VLOperand = Node->getOperand(1);
 
     if (auto *C = dyn_cast<ConstantSDNode>(VLOperand)) {
       uint64_t AVL = C->getZExtValue();
       if (isUInt<5>(AVL)) {

llvm/test/CodeGen/RISCV/rvv/vsetvli-intrinsics.ll

 ; CHECK-NEXT:    vsetvli zero, a0, e32, m2, ta, ma
 ; CHECK-NEXT:    vle32.v v8, (a1)
 ; CHECK-NEXT:    ret
   %vl0 = call iXLen @llvm.riscv.vsetvli.iXLen(iXLen %avl, iXLen 2, iXLen 1)
   %vl1 = call iXLen @llvm.riscv.vsetvli.iXLen(iXLen %vl0, iXLen 2, iXLen 1)
   %x = call <vscale x 4 x i32> @llvm.riscv.vle.nxv4i32.iXLen(<vscale x 4 x i32> undef, <vscale x 4 x i32>* %ptr, iXLen %vl1)
   ret <vscale x 4 x i32> %x
 }
+
+define iXLen @test_vsetvli_negone_e8m1(iXLen %avl) nounwind {
+; CHECK-LABEL: test_vsetvli_negone_e8m1:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    vsetvli a0, zero, e8, m1, ta, ma
+; CHECK-NEXT:    ret
+  %vl = call iXLen @llvm.riscv.vsetvli.iXLen(iXLen -1, iXLen 0, iXLen 0)
+  ret iXLen %vl
+}