nir/large_constants: De-duplicate constants

If a function has a constant and is called more than once, after
inlining we may end up with different variables representing the same
constant.  This commit look into the data and de-duplicate them.

The first pass now will collect the constant data in a per variable
buffer, then de-duplication happens (by sorting then linear walk), and
the second pass will use the data in var->data.location.

One side-effect of the current implementation is that constants will
be reordered.  If this turns out to be a problem is something that can
be fixed.

An alternative strategy considered was to perform this in a
per-function basis and then merge the results, the problem is that we
would have to fix up the offsets during the merge.  Given the data we
have, the current patch is good enough.

Reviewed-by: Jason Ekstrand <[email protected]>

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