glsl: Accept nonconstant array references in lower_ubo_reference

glsl: Accept nonconstant array references in lower_ubo_reference

Instead of falling back to just the block name (which we won't find),
look for the first element of the block array. We'll deal with the rest
in the backend by arranging for the blocks to be laid out contiguously.

V2: Squashed together patches 3, 5 of V1, plus a naming tweak.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>