[RISCV] Remove an unnecessary copy of X0 in selectShiftMask.

We know which instruction we're emitting so its ok to directly
encode X0 into the instruction. We only need to create a copy when
a constant 0 is selected without context of what instructions uses it.

Author: topperc

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

@@ -1942,8 +1942,7 @@ bool RISCVDAGToDAGISel::selectShiftMask(SDValue N, unsigned ShiftWidth,
     if (Imm != 0 && Imm % ShiftWidth == 0) {
       SDLoc DL(N);
       EVT VT = N.getValueType();
-      SDValue Zero =
-          CurDAG->getCopyFromReg(CurDAG->getEntryNode(), DL, RISCV::X0, VT);
+      SDValue Zero = CurDAG->getRegister(RISCV::X0, VT);
       unsigned NegOpc = VT == MVT::i64 ? RISCV::SUBW : RISCV::SUB;
       MachineSDNode *Neg = CurDAG->getMachineNode(NegOpc, DL, VT, Zero,
                                                   N.getOperand(1));