glsl: Use constant_expression_value instead of as_constant

Just a few lines earlier we may have wrapped the index expression with
ir_unop_i2u expression.  Whenever that happens, as_constant will return
NULL, and that almost always happens.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>

src/glsl/lower_ubo_reference.cpp

@@ -219,7 +219,8 @@ lower_ubo_reference_visitor::handle_rvalue(ir_rvalue **rvalue)
          if (array_index->type->base_type == GLSL_TYPE_INT)
             array_index = i2u(array_index);
 
-	 ir_constant *const_index = array_index->as_constant();
+	 ir_constant *const_index =
+            array_index->constant_expression_value(NULL);
 	 if (const_index) {
 	    const_offset += array_stride * const_index->value.u[0];
 	 } else {