Add xxsel

Add xxsel

This patch adds the VSX Select (xxsel) instruction.

The xxsel instruction has four VSR operands.  Thus the xC
instruction decoder is added.

The xxsel instruction is massively overloaded in the opcode
table since only bits 26 and 27 are opcode bits.  This
overloading is done in matrix fashion with two macros
(GEN_XXSEL_ROW and GEN_XX_SEL).

V2: (1) eliminated unecessary XXSEL macro  (2) tighter implementation
using tcg_gen_andc_i64.

Signed-off-by: Tom Musta <tommusta@gmail.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Alexander Graf <agraf@suse.de>