nir+vtn: vec8+vec16 support

nir+vtn: vec8+vec16 support

This introduces new vec8 and vec16 instructions (which are the only
instructions taking more than 4 sources), in order to construct 8 and 16
component vectors.

In order to avoid fixing up the non-autogenerated nir_build_alu() sites
and making them pass 16 src args for the benefit of the two instructions
that take more than 4 srcs (ie vec8 and vec16), nir_build_alu() is has
nir_build_alu_tail() split out and re-used by nir_build_alu2() (which is
used for the > 4 src args case).

v2 (Karol Herbst):
  use nir_build_alu2 for vec8 and vec16
  use python's array multiplication syntax
  add nir_op_vec helper
  simplify nir_vec
  nir_build_alu_tail -> nir_builder_alu_instr_finish_and_insert
  use nir_build_alu for opcodes with <= 4 sources
v3 (Karol Herbst):
  fix nir_serialize
v4 (Dave Airlie):
  fix serialization of glsl_type
  handle vec8/16 in lowering of bools
v5 (Karol Herbst):
  fix load store vectorizer

Signed-off-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>