[Mesa-dev] draw: simplify overflow handling, unify elts and linear jit code

[sroland at vmware.com]

Overflow handling is simplified quite a bit both in jit code and vsplit
paths (basically just let things wrap around everywhere). This seems to
be good enough for all apis.
Also, elts and linear jit code is unified since the differences are minimal
(even more so at the end of the series). The cost of having 2 different jit
shaders is definitely not worth it (the combined shader isn't really more
complex than the elt one was, with the difference to the linear one easily
being skipped for the most part for the linear case).
(These patches are on top of the last series.)