draw: use vectorized calculations for fetch (v2)

Instead of doing all the math with scalars, use vectors. This means the
overflow math needs to be done manually, albeit that's only really
problematic for the stride/index mul, the rest has been pretty much
moved outside the shader loop (albeit the mul could actually be optimized
away too), where things are still scalar.
To eliminate control flow in the main shader loop fetch, provide fake
buffers (so index 0 is always valid to fetch).
Still uses aos fetch though in the end - mostly because some more code
would be needed to handle unaligned fetches in that path, and because for
most formats it won't make a difference anyway (we generate some truly
horrendous code for things like R16G16_something for instance).

Instanced fetch however stays roughly the same as before, except that
no longer the same element is fetched multiple times (I've seen a reduction
of ~3 times in main shader loop size due to llvm not recognizing it's all
the same fetch, since it would have been possible some of the fetches
getting replaced with zeros in case vector size exceeds remaining fetch
count - the values of such fetches don't matter at all though).

Also, for elts gathering, use vectorized code as well.

The generated shaders are smaller and faster to compile (not entirely sure
about execution speed, but generally unless there's just single vertices
to handle I would expect it to be faster - there's more opportunities
for future improvements by using soa fetch).

v3: skip the fake index buffer, not needed due to the jit code never seeing
the real index buffer in the first place.
Fix a bug with mask expansion (needs SExt, not ZExt).
Also, be really really careful to keep the behavior the same, even in cases
where it looks wrong, and add comments why the code is doing the seemingly
wrong stuff... Fortunately it's not actually more complex in the end...
Also change function order slightly just to make the diff more readable.

No piglit change. Passes some internal testing with another api too...

Reviewed-by: Jose Fonseca <[email protected]>

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