nir: Add a NIR_VALIDATE environment variable

nir: Add a NIR_VALIDATE environment variable

It defaults to true so default behavior doesn't change but it allows you to
do NIR_VALIDATE=false if you don't want validation.  Disabling validation
can substantially speed up shader compiles so you frequently want to turn
it off if compiler invariants aren't in question.
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Rob Clark <robclark@freedesktop.org>