ref: a0d56750847f36ffc56a813eec79c31de8fefeeb 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