[X86] Add a new format type for instructions that represent named prefix bytes like...

[X86] Add a new format type for instructions that represent named prefix bytes like data16 and rep. Use it to make a simpler version of isPrefix.

isPrefix was added to support the patches to align branches.
it relies on a switch over instruction names.

This moves those opcodes to a new format so the information is
tablegen and we can just check for a specific value in some bits
in TSFlags instead.

I've left the other function in place for now so that the
existing patches in phabricator will still work. I'll work with
the owner to get them migrated.