glsl: Make ir_dereference_array constructor assert the variable exists.

glsl: Make ir_dereference_array constructor assert the variable exists.

Providing a NULL pointer to the ir_dereference_array() constructor seems
like a bad idea.  Currently, if provided NULL, it returns a partially
constructed value of error type.  However, none of the callers are
prepared to handle that scenario.

Code inspection shows that all callers do one of the following:
- Already NULL-check the argument prior to creating the dereference
- Already deference the argument (and thus would crash if it were NULL)
- Newly allocate the argument.

Thus, it should be safe to simply assert the value passed is not NULL.
This should also catch issues right away, rather than dying later.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>