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 <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>
Reviewed-by: Eric Anholt <96f164ad4d9b2b0dacf8ebee2bb1eeb3aa69adf1@anholt.net>

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