For vector instructions, the helpers get pointers to the vector register
in arguments. Some operands might point to the same register, including
the operand holding the result.

When emulating instructions which access the vector elements in a
non-linear way, we need to store the result in an temporary variable.

This fixes openssl when emulating a POWER8 CPU.
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>
Reviewed-by: NRichard Henderson <rth@twiddle.net>
Signed-off-by: NAlexander Graf <agraf@suse.de>