gallivm: implement better control of per-quad/per-element/scalar lod

gallivm: implement better control of per-quad/per-element/scalar lod

There's a new debug value used to disable per-quad lod optimizations
in fragment shader (ignored for vs/gs as the results are just too wrong
typically). Also trying to detect if a supplied lod value is really a
scalar (if it's coming from immediate or constant file) in which case
sampler code can use this to stay on per-quad-lod path (in fact for
explicit lod could simplify even further and use same lod for both
quads in the avx case but this is not implemented yet).
Still need to actually implement per-element lod bias (and derivatives),
and need to handle per-element lod in size queries.

v2: fix comments, prettify.

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