x86: make symbol quotation check consistent in i386_att_operand()

x86: make symbol quotation check consistent in i386_att_operand()

When d02603dc201f ("Allow symbol and label names to be enclosed in
double quotes") added the check for a leading double quote to
i386_att_operand(), it missed a second similar check after having found
a segment override. To avoid the two checks going out of sync again,
introduce an inline helper.