target-i386: SSE4.2: fix pcmpXstrX instructions with "Masked(-)" polarity

valids can equals to -1 if the reg/mem string is empty. Change the
expression to have an empty xor mask in that case.

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