glsl: Make ir_dereference_variable ctor assert the variable exists.

glsl: Make ir_dereference_variable ctor assert the variable exists.

This also seems like a bad idea.  There were too many instances for me
to thoroughly scan the code as I did with the last two patches, but a
quick scan indicated that most callers newly allocate a variable,
dereference it, or NULL-check.  In some cases, it wasn't clear that the
value would be non-NULL, but they didn't check for error_type either.

At any rate, not checking for this is a bug, and assertions will trigger
it earlier and more reliably than returning error_type.

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