[RISCV] Add OPCFG format of vector. NFC

Refer to https://github.com/riscv/riscv-v-spec/blob/master/v-spec.adoc#101-vector-arithmetic-instruction-encoding

Patch by Jiejie Rong

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D137694

Author: topperc

llvm/lib/Target/RISCV/RISCVInstrFormatsV.td

@@ -20,6 +20,7 @@ def OPIVI : RISCVVFormat<0b011>;
 def OPIVX : RISCVVFormat<0b100>;
 def OPFVF : RISCVVFormat<0b101>;
 def OPMVX : RISCVVFormat<0b110>;
+def OPCFG : RISCVVFormat<0b111>;
 
 class RISCVMOP<bits<2> val> {
   bits<2> Value = val;