util/prim: clean up and add comments

util/prim: clean up and add comments

Move together (or add) functions to decompose/reduce/assemble a primitive,
give them consistent names, and document them.  Add u_prim_vertex_count() so
that the vertex count information can be used elsewhere.

u_assembled_primitive() will be removed in a folow-on commit.

[olv: fix a warning when -Wold-style-declaration is enabled]

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Acked-by: Zack Rusin <zackr@vmware.com>