gallivm,llvmpipe: handle TXF (texelFetch) instruction, including offsets

gallivm,llvmpipe: handle TXF (texelFetch) instruction, including offsets

This also adds some code to handle per-quad lods for more than 4-wide fetches,
because otherwise I'd have to integrate the texelFetch function into
the splitting stuff... (but it is not used yet outside texelFetch).
passes piglit fs-texelFetch-2D, fails fs-texelFetchOffset-2D due to I believe
a test error (results are undefined for out-of-bounds fetches, we return
whatever is at offset 0, whereas the test expects [0,0,0,1]).
Texel offsets are only handled by texelFetch for now, though the interface
can handle it for everything.

Reviewed-by: José Fonseca <jfonseca@vmware.com>