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 <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>

Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>
Signed-off-by: Rob Clark <6f78cdb10adbe0a7fbbf4098125a51f401c77a5f@freedesktop.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler