glsl: fix lowering of UBO references of named blocks

glsl: fix lowering of UBO references of named blocks

When a UBO reference has the form block_name.foo where block_name refers
to a block where the first member has a non-zero offset, the base offset
was incorrectly added to the reference.

Fixes an assertion triggered in debug builds by
GL45-CTS.enhanced_layouts.uniform_block_layout_qualifier_conflict. That test
doesn't properly check for correct execution in this case, so I am also
going to send out a piglit test.

Cc: 13.0 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>