[RISCV] Add OPCFG format and use it for vsetvli. NFC

[craig.topper]

Summary

Based on D137694 and extended

[kito-cheng]

LGTM

[JojoR]

LGTM also

llvm/lib/Target/RISCV/RISCVInstrFormatsV.td

class RVInstSetiVLi<dag outs, dag ins, string opcodestr, string argstr>
   bits<5> uimm;
   bits<5> rd;
   bits<10> vtypei;
 
   let Inst{31} = 1;
   let Inst{30} = 1;
   let Inst{29-20} = vtypei{9-0};
   let Inst{19-15} = uimm;
-  let Inst{14-12} = 0b111;
+  let Inst{14-12} = OPCFG.Value;
   let Inst{11-7} = rd;
   let Opcode = OPC_OP_V.Value;
 
   let Defs = [VTYPE, VL];
 }
 
 class RVInstSetVLi<dag outs, dag ins, string opcodestr, string argstr>
     : RVInst<outs, ins, opcodestr, argstr, [], InstFormatI> {
   bits<5> rs1;
   bits<5> rd;
   bits<11> vtypei;
 
   let Inst{31} = 0;
   let Inst{30-20} = vtypei;
   let Inst{19-15} = rs1;
-  let Inst{14-12} = 0b111;
+  let Inst{14-12} = OPCFG.Value;
   let Inst{11-7} = rd;
   let Opcode = OPC_OP_V.Value;
 
   let Defs = [VTYPE, VL];
 }
 
 class RVInstSetVL<dag outs, dag ins, string opcodestr, string argstr>
     : RVInst<outs, ins, opcodestr, argstr, [], InstFormatR> {
   bits<5> rs2;
   bits<5> rs1;
   bits<5> rd;
 
   let Inst{31} = 1;
   let Inst{30-25} = 0b000000;
   let Inst{24-20} = rs2;
   let Inst{19-15} = rs1;
-  let Inst{14-12} = 0b111;
+  let Inst{14-12} = OPCFG.Value;
   let Inst{11-7} = rd;
   let Opcode = OPC_OP_V.Value;
 
   let Defs = [VTYPE, VL];
 }
 
 class RVInstVV<bits<6> funct6, RISCVVFormat opv, dag outs, dag ins,
                string opcodestr, string argstr>