target-ppc: add lxvh8x instruction

target-ppc: add lxvh8x instruction

lxvh8x:  Load VSX Vector Halfword*8

Big-Endian Storage
+-------+-------+-------+-------+-------+-------+-------+-------+
| 00 01 | 10 11 | 20 21 | 30 31 | 40 41 | 50 51 | 60 61 | 70 71 |
+-------+-------+-------+-------+-------+-------+-------+-------+

Little-Endian Storage
+-------+-------+-------+-------+-------+-------+-------+-------+
| 01 00 | 11 10 | 21 20 | 31 30 | 41 40 | 51 50 | 61 60 | 71 70 |
+-------+-------+-------+-------+-------+-------+-------+-------+

Vector load results in (16-bit elements):
+------+------+------+------+------+------+------+------+
| 0001 | 1011 | 2021 | 3031 | 4041 | 5051 | 6061 | 7071 |
+------+------+------+------+------+------+------+------+

Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
[dwg: Tweak to commit description]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>