target/s390x: improve MOVE LONG and MOVE LONG EXTENDED

As MVCL and MVCLE only differ by their operands, use a common
do_mvcl helper. Optimize it calling fast_memmove and fast_memset.
Correctly write back addresses. Check that r1 and r2/r3 registers
are even.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Message-Id: <20170531220129.27724-21-aurelien@aurel32.net>
Signed-off-by: Richard Henderson <rth@twiddle.net>
