nir: add comment for loop_unroll pass

Save the next person from digging through the code to figure out what
the indirect_mask parameter actually does.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>

src/compiler/nir/nir_opt_loop_unroll.c

@@ -578,6 +578,10 @@ nir_opt_loop_unroll_impl(nir_function_impl *impl,
    return progress;
 }
 
+/**
+ * indirect_mask specifies which type of indirectly accessed variables
+ * should force loop unrolling.
+ */
 bool
 nir_opt_loop_unroll(nir_shader *shader, nir_variable_mode indirect_mask)
 {