ref: d4f1772327e14809d4874b8c6aa7da261b65e43f dir: /src/cmd/as/target/x86/opers.dat/
reg8 AREG_R8CLASS reg16 AREG_R16CLASS reg32 AREG_R32CLASS imm8 AIMM8 imm16 AIMM16 imm32 AIMM32 imm64 AIMM64 \(n\) ADIRECT sym ASYM string ASTR