form field in target specific td Instruction classes

Most ports pass a format code down to various instructions formats.

This ends up getting packed into TSFlags via FormBits.

The Mips compiler does not really use this at all except for one place in the old style jit but I think
that that code can be eliminated and replaced with something simpler.

Is this just an old remnant or are other people using this too?

I'm adding a new port to Mips and wondering if I should be duplicating this.