glsl: Convert uniform_block in lower_ubo_reference to ir_rvalue.

glsl: Convert uniform_block in lower_ubo_reference to ir_rvalue.

Previously this was a block index with special semantics for -1.
With ARB_gpu_shader5, this need not be a compile-time constant, so
allow any rvalue here and convert the -1 to a NULL pointer.

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