pan/midgard: Add class check

pan/midgard: Add class check

This ensures the rules for accessing special register classes are
satisfied. This is asserted as a prepass should have lowered offending
uses to something satisfying these rules. Special register classes are
*not* work registers and cannot be used for RMW operations; they are
essentially 1-way pipes straight into/from fixed-function logic in the
shader cores.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>