glsl: Make opt_constant_variable() bail in useless cases.

glsl: Make opt_constant_variable() bail in useless cases.

The pass ultimately skips over any entries with assignment_count != 1,
so there's no need to do further work once we've determined that there
are multiple assignments.

The constant value could be a large array (i.e. uvec4[327]), at which
point skipping the constant_expression_value() call (and the clone()
call within) can save us piles of memory.

No change in shader-db.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>