tgsi: add BALLOT/READ_* opcodes

tgsi: add BALLOT/READ_* opcodes

v2 (Nicolai):
- BALLOT isn't per-channel
- expand the documentation (also for VOTE_*)

v3:
- only BALLOT returns a 64-bit lanemask (Boyan)
- relax the requirement on READ_INVOC: the invocation number to read
  from must be uniform within a sub-group. This matches the
  GL_ARB_shader_ballot spect (and the v_readlane instruction of AMD
  GCN)

v4:
- hopefully really fix the doc of VOTE_* returns (Ilia)

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com> (v2)