[RISCV] Use PseudoInstExpansion for PseudoReadVLENB and PseudoReadVL. NFC

This lets tablegen generated the code and avoids a string lookup
of the CSR name at runtime.

llvm/lib/Target/RISCV/RISCVInstrInfoVPseudos.td

@@ -5427,12 +5427,14 @@ let Predicates = [HasVInstructions] in {
 let hasSideEffects = 0, mayLoad = 0, mayStore = 0, isCodeGenOnly = 1 in {
   def PseudoReadVLENB : Pseudo<(outs GPR:$rd), (ins),
                                [(set GPR:$rd, (riscv_read_vlenb))]>,
+                        PseudoInstExpansion<(CSRRS GPR:$rd, SysRegVLENB.Encoding, X0)>,
                         Sched<[WriteRdVLENB]>;
 }
 
 let hasSideEffects = 0, mayLoad = 0, mayStore = 0, isCodeGenOnly = 1,
     Uses = [VL] in
-def PseudoReadVL : Pseudo<(outs GPR:$rd), (ins), []>;
+def PseudoReadVL : Pseudo<(outs GPR:$rd), (ins), []>,
+                   PseudoInstExpansion<(CSRRS GPR:$rd, SysRegVL.Encoding, X0)>;
 
 foreach lmul = MxList in {
   foreach nf = NFSet<lmul>.L in {

llvm/lib/Target/RISCV/RISCVMCInstLower.cpp

@@ -253,18 +253,6 @@ bool llvm::lowerRISCVMachineInstrToMCInst(const MachineInstr *MI, MCInst &OutMI,
     }
     break;
   }
-  case RISCV::PseudoReadVLENB:
-    OutMI.setOpcode(RISCV::CSRRS);
-    OutMI.addOperand(MCOperand::createImm(
-        RISCVSysReg::lookupSysRegByName("VLENB")->Encoding));
-    OutMI.addOperand(MCOperand::createReg(RISCV::X0));
-    break;
-  case RISCV::PseudoReadVL:
-    OutMI.setOpcode(RISCV::CSRRS);
-    OutMI.addOperand(
-        MCOperand::createImm(RISCVSysReg::lookupSysRegByName("VL")->Encoding));
-    OutMI.addOperand(MCOperand::createReg(RISCV::X0));
-    break;
   }
   return false;
 }

llvm/lib/Target/RISCV/RISCVSystemOperands.td

@@ -352,7 +352,7 @@ def : SysReg<"vstart", 0x008>;
 def : SysReg<"vxsat", 0x009>;
 def : SysReg<"vxrm", 0x00A>;
 def : SysReg<"vcsr", 0x00F>;
-def : SysReg<"vl", 0xC20>;
+def SysRegVL : SysReg<"vl", 0xC20>;
 def : SysReg<"vtype", 0xC21>;
 def SysRegVLENB: SysReg<"vlenb", 0xC22>;