ref: b9d7c95aa5bf9e06fc7d6a58cd5d0c600f128bf3 dir: /src/cmd/as/target/x86/rules.dat/
reg8 AREG_R8CLASS reg16 AREG_R16CLASS reg32 AREG_R32CLASS imm8 AIMM8 imm16 AIMM16 imm32 AIMM32 imm64 AIMM64 \(n\) ADIRECT sym ASYM string ASTR