tcg/x86_64: Avoid unnecessary REX.B prefixes.
The existing P_REXB internal opcode flag unconditionally emits the REX prefix. Technically it's not needed if the register in question is %al, %bl, %cl, %dl. Eliding the prefix requires splitting the P_REXB flag into two, in order to indicate whether the byte register in question is in the REG or the R/M field. Within TCG, the byte register is in the REG field only for stores. Signed-off-by: NRichard Henderson <rth@twiddle.net> Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>
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