i965/nir: Validate that NIR passes call nir_metadata_preserve().

i965/nir: Validate that NIR passes call nir_metadata_preserve().

Failing to call nir_metadata_preserve() can have nasty consequences:
some pass breaks dominance information, but leaves it marked as valid,
causing some subsequent pass to go haywire and probably crash.

This pass adds a simple validation mechanism to ensure passes handle
this properly.  We add a new bogus metadata flag that isn't used for
anything in particular, set it before each pass, and ensure it *isn't*
still set after the pass.  nir_metadata_preserve will reset the flag,
so correct passes will work, and bad passes will assert fail.

(I would have made these functions static inline, but nir.h is included
in C++, so we can't bit-or enums without lots of casting...)

Thanks to Dylan Baker for the idea.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>