[RISCV] Select vl op to X0 when it is equal to ~0.

Now the backend will select ~0 vl to a register and load instruction, we could use X0 to replace it.

Differential Revision: https://reviews.llvm.org/D116798

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

@@ -1793,7 +1793,8 @@ bool RISCVDAGToDAGISel::hasAllNBitUsers(SDNode *Node, unsigned Bits) const {
 // allows us to choose betwen VSETIVLI or VSETVLI later.
 bool RISCVDAGToDAGISel::selectVLOp(SDValue N, SDValue &VL) {
   auto *C = dyn_cast<ConstantSDNode>(N);
-  if (C && isUInt<5>(C->getZExtValue()))
+  if (C && (isUInt<5>(C->getZExtValue()) ||
+            C->getSExtValue() == RISCV::VLMaxSentinel))
     VL = CurDAG->getTargetConstant(C->getZExtValue(), SDLoc(N),
                                    N->getValueType(0));
   else

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

@@ -242,8 +242,7 @@ entry:
 define <vscale x 1 x double> @test13(<vscale x 1 x double> %a, <vscale x 1 x double> %b) nounwind {
 ; CHECK-LABEL: test13:
 ; CHECK:       # %bb.0: # %entry
-; CHECK-NEXT:    li a0, -1
-; CHECK-NEXT:    vsetvli zero, a0, e64, m1, ta, mu
+; CHECK-NEXT:    vsetvli a0, zero, e64, m1, ta, mu
 ; CHECK-NEXT:    vfadd.vv v8, v8, v9
 ; CHECK-NEXT:    ret
 entry: