x86: fold MOV to/from segment register templates

x86: fold MOV to/from segment register templates

First of all there's no point in having separate Cpu386 templates - the
respective SReg3 registers can't be specified for pre-386 anyway; see
parse_real_register().

And then we can also make use of D here for the memory forms of the
insn. This cannot be done for the non-64bit GPR forms because of the
IgnoreSize that cannot be dropped from the to-SREG variant.