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>