gallium: simplify tgsi tokens further

gallium: simplify tgsi tokens further

Drop anonymous 'Extended' fields, have every optional token named
explicitly in its parent.  Eg. there is now an Instruction.Label flag,
etc.

Drop destination modifiers and other functionality which cannot be
generated by tgsi_ureg.c, which is now the primary way of creating
shaders.

Pull source modifiers into the source register token, drop the second
negate flag.  The source register token is now full - if we need to
expand it, probably best to move all of the modifiers to a new token
and have a single flag for it.