tcg-i386: Tidy non-immediate arithmetic operations.

tcg-i386: Tidy non-immediate arithmetic operations.

Add more OPC values, and tgen_arithr.  Use the later throughout.

Note that normal reg/reg arithmetic now uses the Gv,Ev opcode form
instead of the Ev,Gv opcode form used previously.  Both forms
disassemble properly, and so there's no visible change when diffing
log files before and after the change.  This change makes the operand
ordering within the output routines more natural, and avoids the need
to define an OPC_ARITH_EvGv since a read-modify-write with memory is
not needed within TCG.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>