x86: re-order insn template fields

This saves quite a number of shift instructions: The "operands" field
can now be retrieved by just masking (no shift), and extracting the
"extension_opcode" field now only requires a (signed) right shift, with
no prereq left one. (Of course there may be architectures where, in a
cross build, there might be no difference at all, e.g. when there are
suitable bitfield extraction insns.)